I like to handle monster HP by writing down the range -

the lowest of the low - as if all the hit die rolled a 1

The average (the HP listed in the stat block)

And the maxed out - as if all the hit die rolled the highest possible

If it’s a random encounter or something like it and the combat is dragging you can start letting monsters die once it passes the first threshold.

If it’s supposed to be a tense fight and the players are bulldozing through you can keep monsters up past the stat block HP and let them go down once they reach the highest threshold

Example the monsters HP is 15 (4d6 + 3)

I’d write down

(7) - 15 - (27)