K it seems to be working now. I tried reflashing both the sketches before, now I restarted win, re-flashed everything once more and.... voilà..... working like a champ. Now I'm running the compensation tool/command/whatever (button 3 on the GUI) but except the red dot went a bit off the little circle everything's smooth so far.
In any case once I'm ingame if I'm not happy with the calibration what should I do? Do I have to reflash the calib sketch, do the 9-button-thingy, reflash the main one and all the rest again?
You shouldn't need to reflash or anything now. At first, I had a bit of drift in game but another couple of runs through the drift compensation process sorted that. As you've done before...let it sit for a while (10 mins), press 3 to adjust the compensation and then 8? to save it. I did mine a few times and ended up with it very stable.