Sale 1:

Sold for 43 keys

Seller: https://backpack.tf/profiles/76561197964171516#!/compare/1553299200/1553385600

Buyer: https://backpack.tf/profiles/76561197994783413#!/compare/1553299200/1553385600

https://backpack.tf/item/5776851580

X = 43

Sale 2:

Sold for 18 Uncraftable white paint cans (1.85*18)

Seller: https://backpack.tf/profiles/76561198100912739#!/compare/1551312000/1551484800

Buyer: https://backpack.tf/profiles/76561198051798340#!/compare/1551398400/1551484800

https://backpack.tf/item/7523419932

https://bot.tf/stats/item?def=5039&q=6&ef=&craft=0&aus=0&ks=0

https://i.imgur.com/8Caq0kC.png - white paint appears to have gone for ~1.8-1.9 keys at the time (~1.85), taking 1.85 per paint.

X = 33

Sale 3:

Sold for 30 keys + 26 ref

Seller: https://backpack.tf/profiles/76561198148843171#!/compare/1551139200/1551225600

Buyer: https://backpack.tf/profiles/76561198100912739#!/compare/1551139200/1551225600

https://backpack.tf/item/7523419932

X = 31

Marketplace Sales:

https://marketplace.tf/items/30311;5;u7

https://manic.tf/keyprice/?at=2019-03-07 - $78.21 / $1.79 = 43.69 keys

X = 44

Sales at:

31

33

43

44

No current sellers.

Going with 31-44.