ROI does not consider the length of the investment. Without keeping the time frame the same across all calculations, your comparisons will be all off.
Once you start using ROI as a comparison tool, you need to use it across all your investments. This also applied to costs you’re including: have you considered inflation or taxes?
Rounding incorrectly may have caused you to get a C on your math exam, but doing so with ROI can cost you quite a chunk of change. To be as accurate as possible, you need to double-check your calculations.