Using last month.

Sales 1 & 2:

Sold for $22 twice on marketplace.tf (30 aug, spt 2)

https://backpack.tf/item/6478391889 - might be one of them, no outpost

https://backpack.tf/item/6457929272

http://www.tf2outpost.com/trade/29715614 - B/O of 10

http://manic.tf/keyprice/?at=2018-08-30 - $22 / $1.88 = 11.70 keys

http://manic.tf/keyprice/?at=2018-09-02 - $22 / $1.88 = 11.70 keys

X = 12

X = 10 (Capped by B/O)

Sale 3:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198055518037#!/compare/1536019200/1536105600

Buyer: https://backpack.tf/profiles/76561198352969437#!/compare/1536019200/1536105600

https://backpack.tf/item/6516659087

X = 12

Sale 4:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1534636800/1534723200

Buyer: https://backpack.tf/profiles/76561198251982584#!/compare/1534464000/1534723200

https://backpack.tf/item/6458071308

https://i.imgur.com/ws78Ek3.png

X = 10

Sale 5:

Sold for 8 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198128213644#!/compare/1535155200/1535241600

Buyer: https://backpack.tf/profiles/76561198295122993#!/compare/1535068800/1535155200

https://backpack.tf/item/6466774643

X = 8.5

Other Sales:

https://backpack.tf/item/6459708547 (#1)

https://backpack.tf/item/6458071308 (#2 & #3)

https://backpack.tf/item/6507900610 - Not unusual at the time

#1

Untraceable

https://backpack.tf/profiles/76561198259331752#!/compare/1536278400/1536364800

https://backpack.tf/profiles/76561198144776495#!/compare/1536278400/1536364800

#2

Untraceable

https://backpack.tf/profiles/76561198251982584#!/compare/1535932800/1536019200

https://backpack.tf/profiles/76561198363395191#!/compare/1535932800/1536019200

#3

Untraceable

https://backpack.tf/profiles/76561198363395191#!/compare/1536969600/1537056000

https://backpack.tf/item/6458071308 - buyer is currently untraceable

Sellers:

https://i.imgur.com/CC1y0FO.png

https://backpack.tf/item/6459708547 - 9 keys

https://backpack.tf/item/6482428577 - 18 keys

Sales at:

8.5

10 (2x)

12 (2x)

Buyers currently peak at ~7.5 keys, should be fine to keep 8.5.

Going with 8.5-12.