Forget all the math.. lol.. its just going to confuse you...

If you are running 1/10th scale stock 17.5.. the higher the C rating the better! The battery can put out more punch.. If you are running a hot brushless.. then you are going to get your power from you motor.. don't worry too much about the C rating.. You can change your gearing and you will still be fast! When its all about even motors in stock, and your driving skills are there, the extra C rating will be an advantage.

A 1/10th scale motor racing modified class isn't going to pull enough amps to hurt you... so a 20C isn't going to cut out or anything vs. a 40C... given the option for the same money.. go for the higher rating.

Jerome