Maxperson
Morkus from Orkus
*A pun is the highest form of humor.
My wife certainly thinks so. She loves puns.

*A pun is the highest form of humor.
The spell says, "...On a failed save, the target takes 10d6+40 force damage..." Lets say your total is 80 points of damage.
The spell then says, "...If this damage reduces the target to 0 hit points, it is disintegrated..."
'This damage' is 80 points. If you haven't applied all 80, then you haven't applied 'this damage'.
Now consider someone who isn't polymorphed, and who has 75 hit points left. You hit them with an 80-point disintegrate. Now what? You apply 75 damage. Now you're done.
No, you don't somehow subtract 5 from the total damage. Someone with 75 hit points gets hit for 80 damage, they got hit for 80 damage. c.f. death from massive damage in the DMG, et al.
No, you don't somehow subtract 5 from the total damage. Someone with 75 hit points gets hit for 80 damage, they got hit for 80 damage. c.f. death from massive damage in the DMG, et al.
No, you don't somehow subtract 5 from the total damage. Someone with 75 hit points gets hit for 80 damage, they got hit for 80 damage. c.f. death from massive damage in the DMG, et al.
It's impossible to deal 80 points of damage to someone with 50 hit points. Once you hit 0, you stop since there are no negative numbers in 5e. Read the Instant Death section. Once you hit 0, damage just remains and doesn't do anything at all unless it equals your maximum hit points. Therefore, disintegrate must be able to ash someone by dealing partial damage. It would be nonsensical to expect that disintegrate could only ash someone if you roll exactly the number of hit points they have left.