Resuggestion, thanks to maGIK & Blaze for the info!

Sale 1:

https://backpack.tf/profiles/76561198928347702#!/compare/1585267200/1585353600

https://backpack.tf/profiles/76561197994095287#!/compare/1578614400/1587513600

untrace

sale 2:

https://backpack.tf/profiles/76561198236985112#!/compare/1586131200/1586217600

https://backpack.tf/profiles/76561198084219943#!/compare/1586131200/1586217600

Sold for ~13 keys

sale 3:

https://backpack.tf/profiles/76561198084219943#!/compare/1586131200/1586217600

https://backpack.tf/profiles/76561198082646826#!/compare/1583107200/1586217600

Sold for 20 keys

sale 4:

https://backpack.tf/profiles/76561198980390771#!/compare/1582156800/1582243200

https://backpack.tf/profiles/76561198068182991#!/compare/1571875200/1584576000

Untrace

sale 5:

https://backpack.tf/profiles/76561198324718706#!/compare/1584057600/1585872000

https://backpack.tf/profiles/76561198033661509#!/compare/1585872000/1586044800

untrace

sale 6:

https://backpack.tf/profiles/76561198332101050#!/compare/1581897600/1581984000

https://backpack.tf/profiles/76561198341171690#!/compare/1581897600/1581984000

Sold for 15 keys (QS)

sale 7:

https://backpack.tf/profiles/76561198341171690#!/compare/1586044800/1586131200

https://backpack.tf/profiles/76561198065927956#!/compare/1585958400/1586131200

Sold for 19 keys

sale 8:

https://backpack.tf/profiles/76561198065927956#!/compare/1586131200/1586304000

https://backpack.tf/profiles/76561198341171690#!/compare/1586217600/1586304000

Untrace / QS

sale 9:

https://backpack.tf/profiles/76561198341171690#!/compare/1588118400/1588204800

https://backpack.tf/profiles/76561198436894675#!/compare/1582416000/1589241600

Untrace

Sale 10:

https://backpack.tf/profiles/76561198200973421#!/compare/1588464000/1588982400

https://backpack.tf/profiles/76561198134220286#!/compare/1588464000/1588550400

Sold for 14 keys

sale 11:

https://backpack.tf/profiles/76561198118334319#!/compare/1582243200/1582416000

https://backpack.tf/profiles/76561198363395191#!/compare/1582329600/1582416000

Sold for ~13 keys (QS)

sale 12:

https://backpack.tf/profiles/76561198363395191#!/compare/1582329600/1582416000

https://backpack.tf/profiles/76561198151833282#!/compare/1582416000/1582934400

untrace

sale 13:

https://backpack.tf/profiles/76561198028422102#!/compare/1589068800/1589241600

https://backpack.tf/profiles/76561198928347702#!/compare/1589068800/1589241600

QS

sale 14:

https://backpack.tf/profiles/76561198324772302#!/compare/1588204800/1588982400

https://backpack.tf/profiles/76561198101062081#!/compare/1588896000/1588982400

Sold for ~7.5 keys in sweets

Sale 15:

https://backpack.tf/profiles/76561198101062081#!/compare/1588982400/1589068800

https://backpack.tf/profiles/76561198179287853#!/compare/1588982400/1589068800

sold for keys

sale 16:

https://backpack.tf/profiles/76561198179287853#!/compare/1589068800/1589155200

https://backpack.tf/profiles/76561198257609313#!/compare/1588723200/1589068800

Sold for keys

Sale 17:

https://backpack.tf/profiles/76561198257609313#!/compare/1589155200/1589241600

https://backpack.tf/profiles/76561198236030772#!/compare/1589155200/1589241600

use there

sale 18:

https://backpack.tf/profiles/76561198236030772#!/compare/1589241600/1589500800

https://backpack.tf/profiles/76561198199930778#!/compare/1589414400/1589500800

Sold for ~12 keys

sale 19:

https://backpack.tf/profiles/76561198421005939#!/compare/1584748800/1584835200

https://backpack.tf/profiles/76561197963199720#!/compare/1584662400/1584748800

Untrace

sale 20:

https://backpack.tf/profiles/76561197963199720#!/compare/1584748800/1584835200

https://backpack.tf/profiles/76561198867626964#!/compare/1583971200/1585008000

Untrace

sale 21:

https://backpack.tf/profiles/76561198410760897#!/compare/1584489600/1584576000

https://backpack.tf/profiles/76561198158058289#!/compare/1584403200/1584576000

Untrace

sale 22:

https://backpack.tf/profiles/76561198324282914#!/compare/1583193600/1583280000

https://backpack.tf/profiles/76561198168863803#!/compare/1583193600/1583280000

Bulk

sale 23:

https://backpack.tf/profiles/76561198168863803#!/compare/1583193600/1583280000

https://backpack.tf/profiles/76561198363395191#!/compare/1583193600/1583280000

QS for 14 keys

sale 24:

https://backpack.tf/profiles/76561198796671544#!/compare/1586044800/1586131200

https://backpack.tf/profiles/76561198363395191#!/compare/1586044800/1586131200

QS

sale 25:

https://backpack.tf/profiles/76561198883188648#!/compare/1582588800/1583625600

https://backpack.tf/profiles/76561198137408503#!/compare/1583539200/1583625600

untrace

sale 26:

https://backpack.tf/profiles/76561198041186431#!/compare/1582416000/1582502400

https://backpack.tf/profiles/76561198928347702#!/compare/1582416000/1582502400

QS

sale 27:

https://backpack.tf/profiles/76561198928347702#!/compare/1582848000/1582934400

https://backpack.tf/profiles/76561198085798401#!/compare/1581552000/1584057600

Untrace

sale 28:

https://backpack.tf/profiles/76561198224956812#!/compare/1586304000/1586390400

https://backpack.tf/profiles/76561198172690987#!/compare/1586390400/1586476800

Sold for 11 keys

Sale 29:

https://backpack.tf/profiles/76561198172690987#!/compare/1586563200/1586649600

https://backpack.tf/profiles/76561198163116031#!/compare/1586649600/1586736000

Sold for 13 keys

sale 30:

https://marketplace.tf/items/tf2/31101;5;u145

$34.95 / $1.77 = 19.75 keys

$40 / $1.76 = 22.73 keys

$34.5 / $1.68 = 20.54 keys <-- https://backpack.tf/item/8537936161

$34.95 / $1.70 = 20.56 keys <-- https://backpack.tf/item/8521928339

$28 / $1.77 = 15.82 keys <-- https://backpack.tf/item/8559986117

$32 / $1.78 = 17.98 keys <-- https://backpack.tf/item/8537936161

$22.01 / $1.78 = 12.37 keys

$32 / $1.80 = 17.78 keys

Total sales @:

11, 12, 12, 13, 13, 13, 14, 14, 15, 16, 18, 18, 19, 20, 20, 21, 21, 23 keys

buyers up to ~11 keys, one high buyer up to 12 keys,

multiple unsolds for 18 keys,

poorly bumped seller at 16 keys,

Magik's Part:

Just updating.

Untrace

https://backpack.tf/profiles/76561198820335875#!/compare/1591401600/1591660800

https://backpack.tf/profiles/76561198803829026#!/compare/1591574400/1591660800

Untrace

https://backpack.tf/profiles/76561198414515070#!/compare/1591315200/1591574400

https://backpack.tf/profiles/76561198820335875#!/compare/1591315200/1591228800

Sold to itembot

https://backpack.tf/profiles/76561198084032131#!/compare/1590710400/1591315200

https://backpack.tf/profiles/76561198363395191#!/compare/1591228800/1591315200

Sold for 18 keys

https://backpack.tf/profiles/76561198928347702#!/compare/1590105600/1590192000

https://backpack.tf/profiles/76561198100910467#!/compare/1590105600/1590710400

Sold for 35.99 on May 31, 2020 (35.99/1.91 = 18.84 rounding to 19 keys)

Sold for 32.99 on June June 7, 2020 (32.99/1.98 = 16.66 rounding to 17 keys)

https://marketplace.tf/items/tf2/31101;5;u145

https://backpack.tf/item/8402815239

Current classifieds:

https://gyazo.com/5fe22a3e17701a28a0c0ec9aa6d318cb

Looking at classifieds it seems like 18 or higher is high, but sales are still happening at higher, which makes it too common to exclude here.

I think the wide range is fine, but that's just me.

Blaz'es Part:

21 keys is looking pretty high now given every single seller on classies/scm/marketplace are all below 20 keys

i would prefer a tighter range here

https://imgur.com/a/IAkRnKN

https://steamcommunity.com/market/listings/440/Unusual%20Missing%20Piece?filter=Pyroland

https://marketplace.tf/items/tf2/31101;5;u145

Given MaGIK's extra info, and that at least 3 of the MPTF sales (20, 21, 23) are out of date or nearly so,

given the 2 unsolds for a month each at 15 and 16, this would conserve at those unsolds.

yeah, 13-18 sounds reasonable to me