I would actually make it 18-19, because, and take it with a grain of salt, I do not know exactly how quicksells are counted into this, I actually bought the one you have in sale 4 and he clearly stated that it was a quicksell and that he wasn't selling it for full price, so that would make it 2 sales at 18, then a sale at 19,45 and 1 sale at 20,45.

Sold here + haunted chicken kiev (~3) for planets tower (~28)

http://backpack.tf/profiles/76561198049166729?time=1447833600&compare=1448006400

I havent looked into it any further, but I came across it while doing the tower, I already felt 18 flat would be too low for this

Tower mini: http://backpack.tf/vote/id/5679e69eba8d880d25a94a64

Also, little advice:

In sale 2 you are stacking overpay (i.e youre doing *0.9 in every step). If you have 3 minis, that means you end up having done *0.9 4 times total, meaning you basicly do the value you get *0.65, causing the sale to end up really low.

It is generally better to do overpay either in your mini or in the actual sale, so you have the 0.9 factor only once. Doing this many minis may result in inaccuracy since you only use one sale per hat. The more minis you need, the higher the probability will be that you use a low or high outlier at some point. If you notice you need a string of minis, it may be best to verify the value with other sales as well. It's not wrong what you do here, but its usually best to look into it a little further.

Sale 2 would end up being ~21 keys and the tower sale would be 28-3 = 25. Seeing how both 18 sales went quite fast, this may call for a range 18-25.