The key here is in figuring out the Amps. It's not something that you can plug into a simple formula on a calculator, or do on a sheet of paper. You've only got three choices when it comes to figuring out Amps:

1. Measure with an Ammeter, Astro Whattmeter, or similar device. Obviously, this can sometimes be a chicken-or-egg situation, especially with brushless controllers. You can't measure the current without an ESC, but you don't know what size ESC to buy until you measure the current...
2. Program the components into a "calc" program like P-Calc or Motocalc. These will get you in the ballpark most of the time.
3. Use the manufacturer's application tables, if they have them.

Worst case scenario, get a controller that can handle the motor's maximum rated current.