Help Calculating Likely Range of ROI

[ThorGoT]

Assume I have a 25% ROI over 500 games (or, perhaps, a 24.34% ROI over 516 games). I know there is a formula for calculating what my real ROI is (with 95% and 65% certainty, or something like that). But I can't seem to locate it using the search function. Plus my math/statistical background is weak. I remember what a square root is, though. Can anyone help? Thanks