For diamond, a rule of thumb I try to stick by is to reduce the pressure used on them by at least 2/3 of whatever pressure you might use with other stones or sharpening tools. Diamond is roughly 2.5-3X as hard as the next-hardest commonly used abrasives in stones (silicon carbide, aluminum oxide mainly), so it'll cut just as aggressively with much, much less pressure. Going too heavy with diamond hones removes far too much metal than is necessary, leaves a very ragged edge that's harder to refine, and it's also not good for the diamond hone itself, as the diamond grit can be ripped out of the nickel substrate of the hone. Learning the proper 'light touch' that works well with diamond will also pay dividends with every other sharpening tool you might use down the road, as a lighter touch will always yield better results in the refining stages, no matter what tool you use.
David