I have a post it on my screen with the %'s needed to call according to pot size.

Villain bets 2x Pot - we need to be good 40% of the time.
Pot - 33%
2/3 Pot - 28%
1/2 Pot - 25%
1/3 Pot - 20%
1/4 Pot - 16%

I got this from a book I read, have pretty much memorised it now but it might help, rather than working everything out you can just memorise it all.