Sale 1:

Sold for 20 keys + ~1 key in metal/items

No outpost

Seller: http://backpack.tf/profiles/76561198063640894#!/compare/1473465600/1473552000

Buyer: http://backpack.tf/profiles/76561198043075767#!/compare/1473552000/1473724800

X = 21

Sale 2:

Sold for Aces Fed Fighting Fedora (20.5)

http://www.tf2outpost.com/trade/28488122 - B/O of 20

Seller: http://backpack.tf/profiles/76561198191980011#!/compare/1474070400/1474156800

Buyer: http://backpack.tf/profiles/76561198077715285#!/compare/1474070400/1474156800

X = 20 (capped by B/O)

Sale 3:

Sold for Cloud 9 Dead Cone (Mini at 15.5 below) + Hellmet

No outpost

Seller: http://backpack.tf/profiles/76561198043075767#!/compare/1473811200/1473897600

Buyer: http://backpack.tf/profiles/76561198191980011#!/compare/1473724800/1473811200

Cloud 9 Dead Cone Mini:

http://backpack.tf/suggestion/581ad2fac44045159555f19d

Sales at:

16

20

21

No current sellers

16 is a bit of an outlier, and would create an unnecessarily large range. Excluding 16

Going with 20-21