Coil loot dropped... won't let me have it.

[waca]

Like I said, there are 17-18 (depending on turn) drops for each turn. There is a 100% chance that one of the loots drop. 100% / 17 (or 18) is between 5-6% chance for each piece.

Now, I am actually asking how do you know the formula should be 100%/17 or 100%/18? This is the source I'm interested in knowing.