[ashlar]

I think it's pretty simple, once you understand the underlying concepts.

First of all you launch a video with the fps you want to fix (the fps is kind of irrelevant, as the clock deviation remains the same no matter the refresh rate the card outputs).
You let it play for 15 minutes or so and look at the clock deviation. This is the important variable to know what refresh rate you'll need to aim for.

The deviation is expressed as a percentage, negative or positive.

I'll make two examples for 23.976 refresh (24/1.001). One with a positive deviation of 0.00242%, one with a negative deviation of 0.00242%

Positive clock deviation (0.00242%)

Target refresh rate = 23.97602 * 1.0000242 = 23.976604

Negative clock deviation (-0.00242%)

Target refresh rate = 23.97602 * 0.9999758 = 23.97544

You basically add two decimal positions (zeroes) to the percentage indicated and then add it (positive deviation) or subtract it (negative deviation) to 1. You then multiply that number for the correct refresh rate, in order to get a target refresh rate that compensates your clock deviation. This target refresh rate is the one you need to try and come up with through custom resolutions.