What's the best way to calculate how many amps your ESC should have? I always just made sure my ESC had a higher amp rating than my motors but I'm sure there is a better explanation

Reason being I need to get a new ESC for my novak 13.5T motor but I can't find how many amps it draws. I bought it as a system so I never really worried about the draw with this system. I'm thinking it probably pulls around 80 amps (of course there are other variables that come into play). So I'm not sure if I should just get a 120amp esc or if that would be overkill.
I'm using a 4200mah 100C battery so 4.2x100C=420 so I'll definitely have enough juice for the motor/esc.
Any help would be appreciated.
Thanks!