It's not 100% clear to me, what are you trying to do? If you want to compare two videos, one being 210 fps and the other 240 fps it should be possible.
First, go to Options > Preferences > Playback > General and uncheck "Link speed sliders when comparing videos".
Next you need to find the slow motion factor you will have to apply to one of the video so that their ratios to real time match. This depends on the playback framerate marked in the file (e.g: 30 fps).
- Video A has a capture framerate of 240 fps, and a playback framerate of 30 fps.
- Video B has a capture framerate of 420 fps, and a playback framerate of 24 fps.
- Let Ra be the ratio to real time of A : 30 / 240 = 0.125.
- Let Rb be the ratio to real time of B : 24 / 420 = 0.0571.
- Let S be the slow down factor to apply to A so that it matches the ratio to real time of B : Rb / Ra = 0.45714.
- By setting video A speed to ~46% of video B speed, you should be able to compare them on equal grounds.
- This speed should be set via the main speed slider.
The formula can be summarized like this:
- slowdown for A = (Ac * Bp)/(Ap * Bc).
- Where Ac and Bc are the capture framerate of A and B, and Ap and Bp are the playback framerate of A and B.
You can read the playback framerate by going to Video > Configure video timing…. In the "Video" section, read the "Framerate read in the file" value.
Please plug your numbers into this and let me know if it works.
I realize this is not trivial, should probably detect this comparison case and offer a way to set this up automatically.