[ashlar]

I think you've confused me.

I have a clock deviation of -0.00001%. This means my video clock is too slow.

I want to increase the video clock repeat interval: 23.976 * (1 - 0.00001) = 23.97576 Hz

I will change values in the custom resolution utility until I get a value from the display close to the reference: 23.97576 Hz?.

In the calculation you have to add two decimal positions, as it's a percentage.

So if madVR is showing a negative clock deviation of 0.00001%, you need to aim for a refresh rate of:

23.976023976 * 0.9999999 = 23.976021578

Are you sure the clock variation isn't 0.001%. In that case you should aim for 23.975784216 refresh rate to compensate.
I ask because with 0.00001% variation you should already be set for no drops/repeats for days (ie. never).