The formulas for determining everything could get quite messy. There are a lot of variables to take into consideration that aren't quantifiable (like drive style, terrain, etc), and the ones that are quantifiable may be difficult to measure for most people (wind resistance, drivetrain drag, etc). So, the best one can do is estimate based on experience and maybe a little math. But without every single variable, even some of the math uses fudge factors.
By system, they mean running the BL system at more than 80% of the max battery rating. So, if you calculate average current draw via runtime and battery capacity, and then estimate burst values (usually ~6-7x average), you can then compute the battery needed for 80% "rule". I like to have a battery where the continuous rating of the battery is equal or more than the brushless motor's burst draw.
So, let's say you have a system where you are drawing an average of 20A with 120A estimated bursts. 120A / 80% = 150A. If you have a 5Ah battery, you then need a C rating of 30. Of course, that assumes the battery C rating and capacity figures are true, which opens up a whole new can of worms.
Sometime soon (hopefully very soon), there will be an easier way to help everyone pick the right components without fear of damaging something due to improper component selection. Just keep an eye on the RCM main site and it will happen.
