ik heb het antwoord al ergens gevonden op het forum:
General rule of thumb I always recommend on manual difficulty selection: Round your GH/s down to the closest power of 2. So 1-2 GH/s = 1. 2-4 = 2, 4-8 = 4, 8-16 = 8, etc. In this case, 108 GH/s would be rounded down to 64, as you've attempted Smiley. Using this method, you'll end up between 15 and 30 shares per minute, depending on how close you were to the power of 2, which also happens to be roughly what most pools use for vardiff.
https://bitcointalk.org/index.php?topic=274023.msg2936086#msg2936086