Yeah, I guess the way it's in the file would work. I just wanted to simplify it a bit. I.e. whenever the target would otherwise get disintegrated, it gets nullified instead. No extra rules, just nullify instead of disintegrate.
The "wouldn't want to keep track of the total amount of damage" comment referred to the new version you suggested: "When a creature suffers damage equal to its normal maximum hit points from the breath weapon ..." That makes it sound like you should keep track of the *total* damage done by the breath weapon. So even if the target had gotten healed in between, one more hit with the breath weapon and *fwoosh*! That would just be way too much bookkeeping.
The "wouldn't want to keep track of the total amount of damage" comment referred to the new version you suggested: "When a creature suffers damage equal to its normal maximum hit points from the breath weapon ..." That makes it sound like you should keep track of the *total* damage done by the breath weapon. So even if the target had gotten healed in between, one more hit with the breath weapon and *fwoosh*! That would just be way too much bookkeeping.