Just a quick question I want to make sure I got right: If your HP is divisible by 16 you're at optimal Leftovers recovery?
Yes you are correct.
For example: 96 / 16 = 6
96 x .0625 = 6 (Leftovers)
Substitute # 1: 96 - 24 = 72 + 6 = 78
Substitute # 2: 78 - 24 = 54 + 6 = 60
Substitute # 3: 60 - 24 = 36 + 6 = 42
Substitute # 4: 42 - 24 = 18 + 6 = 24
You can make 4 Substitutes, and with Leftovers.
However, if you do a number divisible by 16 + 1, you end up with 1 HP after 5 Substitutes, because it would be
Substitute # 1: 97 - 24 = 73 + 6 = 79
Substitute # 2: 79 - 24 = 55 + 6 = 61
Substitute # 3: 61 - 24 = 37 + 6 = 43
Substitute # 4: 43 - 24 = 19 + 6 = 25
Substitute # 5: 25 - 24 = 1 + 6 = 7
So by adding just 1 HP, you create an additional Substitute thanks to Leftovers. Or, if you don't have Leftovers then you still can Substitute 4x down to 1 HP (good for Flail / Reversal).