Now that you have explained it, it makes a bit more sense...
I was missing your point completely and kept focusing on the HP variable and the die variable as BOTH being variants when in fact the die variable is a constant - it is the amount of die's within the HP that is the variable.
Thus trying to divide 100 by a D10 will not always take the same amount of attempts to complete but on the final attempt (something you couldn't predict in attempts until it has happened) it will always come down to 1 in 10
I am sorry, I was confusing the randomness of the attempts to reduce the enemy towards the killing blow as a factor but since it is fairly irrelevant to the point of "what is the chance a monster, facing a D10 dmg die will drop to 0" - it of course has to be 1 in 10.
I was just focusing on the wrong variable, the HP Total when it is completely irrelevant.
Thank you and sorry for wasting your time.