[ashlar]

No no, it was with sound.
I uninstalled GeForce Experience, and repeated the test, and this time it gave me -0.00003% , still low i guess

Are you using Reclock as audio renderer with bistream formats accepted? Because I am seeing pretty much the same thing. Reclock messes up with madVR clock deviation calculations. Try using the internal renderer of MPC-HC in recent builds (it's Sanear, basically), it allows for WASAPI bitstreaming and you could use that to calculate the clock deviation. Once you know the value and set up the refresh rates correctly, you can use Reclock with bitstreaming, as (at least this is how I understand it) Reclock does not mess up with the actual sound/video reproduction, it just stops madVR from calculating correctly (but the calculation is just that, a calculation, the clock deviation is there whether madVR is displaying/calculating it or not).