Using last month.

Sale 1:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198448505909#!/compare/1526083200/1526342400

Buyer: https://backpack.tf/profiles/76561198151387693#!/compare/1525564800/1526342400

Bot.tf confirms: https://i.imgur.com/2N6MoYt.png

X = 14

Sales 2:

Sold for $23 on marketplace.tf

https://marketplace.tf/items/30609;5;u3004

23/1.97 = ~11.7

X = 12

Sale 3:

Sold for 8 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1526688000/1526774400

Buyer: https://backpack.tf/profiles/76561198101131482#!/compare/1526688000/1526774400

Bot.tf confirms: https://i.imgur.com/2N6MoYt.png

X = 8.5

Sale 4:

Sold to a bot for 6 keys + 5.11 ref

Seller: https://backpack.tf/profiles/76561198184303667#!/compare/1525478400/1525564800

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1525478400/1525564800

Bot.tf confirms: https://i.imgur.com/2N6MoYt.png

X = 6

Sale 5:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198156650555#!/compare/1527724800/1527811200

Buyer: https://backpack.tf/profiles/76561198431213638#!/compare/1527724800/1527811200

X = 6

Sale 6:

Sold to a bot for 6 keys + 1 ref

Seller: https://backpack.tf/profiles/76561198333837293#!/compare/1527724800/1527811200

Buyer: https://backpack.tf/profiles/76561198358932637#!/compare/1527638400/1527724800

Bot.tf confirms: https://i.imgur.com/2N6MoYt.png

X = 6

Sellers:

https://i.imgur.com/4nqNdQP.png

https://backpack.tf/item/3597990010

https://backpack.tf/item/5373837042

Sales at:

6 (3x)

8.5

12

14

Sellers at 9 show that anything higher than 9 is no longer achievable. Excluding 12 and 14.

Going with 6-8.5.