Unless you are betting with an advantage, your optimal bet size is actually 0%

2% is ok I guess. If you have an advantage, you are probally not overbetting and if you don't have an advantage, at least you are gambling for low stakes. This figure is used because it's conservative. Less likely to be overbetting, and you can withstand an ugly run before you lose a large chunk of your money.

How serious are you about sportsbetting?