Right click the YouTube video and select “Stats for nerds.” The third line is “Current / Optimal Res” - find the two numbers after the @ and enter them for framerate.
Find the starting point (you can use the , and . keys to find the exact frame). Right
click the video and select “Copy debug info” and paste it for starting frame. Repeat for ending frame.
Other versions of this website don't point this out (even though they have the same problem), but make sure to ALWAYS REFRESH YOUR PAGE when retiming a new run, mainly because of the fps not updating in the equation!