I noticed over the course of the expansion that this is the one element of machinist that seems to consistently annoy people. The solution is simply not to worry about micromanaging your DPS so much. Sometimes you'll get dry spells, sometimes you'll get lucky and get procs over and over - it all evens out in the end. In the grand scheme of things, mechanics like natural damage variance and critical strikes impact your DPS just as much.

Instead of thinking of split slug and clean as 3 separate skills, just think of them as parts of the same basic 1-2-3 combo that you cycle through by default. You'll be much happier if you just take whatever you get without thinking about it too much.