I use this
https://docs.google.com/spreadsheets/d/1wbtIc33K45iU1ScUnkB0PlslJ-eLaJlSZY47sPME2Uk/htmlview?usp=sharing&sle=true#
If you want to know the exact amount just power up a few times -- yes you can waste some dust/candy doing it but if it's already showing range of like 95%-97% the amount of dust/candy wasted is negligible.