For most of the bettors that I know, including myself, our definition of ROI isn't return on initial investment, it's return on each dollar invested. Everytime you place a bet at risk, it's an investment. So the difference here is time horizon. Your definition is just the risk of initial bankroll. The other definition is calculating risk of money continuously. This makes more sense to me because if you double your initial investment, it's now part of your bankroll, and you can cash it out whenever you want. If you continue to play, you are now risking your money, which is doubled. Under the "initial bankroll risk" definition, anything above your initial BR is free money and not actually risked.
Under the continuous definition of ROI, 10% for each dollar wagered is very, very good.
Under the continuous definition of ROI, 10% for each dollar wagered is very, very good.