Base your thickness on how "flat" your heatsink/cpu are.
I do this:
Clean off existing grease(of course, if you start fresh with a grease, it is better, since it is impossible to remove all of an old grease).
I use the "finger in a ziploc bag" method. I get really thin, even layers(you should get even layers regardless of thickness).
Then mount your heatsink with minimal twisting or pushign, or tilting.
Mike