If nothing else, being a DM makes you get pretty good at super quick math.
But in a big combat, to make your life easier, try counting HP up instead of down.
When I slot monsters into initiative, I write their AC, name, and max HP. And next to that, I’ll just start tallying up PC damage.
Quick subtraction gets easy, but quick addition is always easier!
Give it a shot 🙂
Tome of Beasts II Kickstarter – Kobold Press
Glen Thompson’s DM Screen – Dyson’s Dodecahedron
198: World Building; Introducing Worlds (feat. Beadle & Grimm’s) – Dungeon Master’s Block
Vault of Magic II – DMsGuild – $6.95
Cursed Classes – DMsGuild – $12.95
Now get out there and tell a story!