With DC Rest pause sets, each segment (Set or mini-set) is taken to failure with a load such that your rep total falls within the range you've set out for yourself (e.g., 20-30 reps total for an DB pressing movement). You'd do the three sets to failure, and if reps total more than 30, the load goes up the next workout you're using that exercise: You don't do another set if you fall short of 30 reps, although that's a strategy that can be used for a cluster set, just not with the standard DC RP set. (John M. uses that strategy in his programs, though.) If you get to 30 reps total in haven't hit a failure point, then the last set is taken to failure (for the stimulus as well as to gauge the appropriate load for the next go 'round).
As far as rep ranges for DB's, going with a HIGHER number is gong to make sense (and likely never getting down to a total much below 15 reps), because the energy cost and danger (literally) in trying to hoist the DB's up for the 2nd a/o third part of the RP set is pretty high. (You can turn what would have been maybe 3 reps into a single plus a failed rep b/c it's such a PITA to get the DB's into place.) Someone who can press the 150's for 8 reps could easily put themselves in a precarious position trying to hoist them up for the 3rd segment of a DC RP set when already fatigued. When training alone, it's "dangerous" enough b/c that last set would be only a double, on top of a somewhat awkward deadlift (straddling a bench, for instance) to get the DB's into position.
You can remedy the above (although there's really no need to go so heavy with DB - progression is the name of the game here, and doing so in higher as well as lower rep ranges which could be done with those exercises that better lend themselves to this) with training partners to hand off the DB's, a DB stand or, my favorite, using DB Powerhooks (if you can find them) and/or connecting daisy chains (nylon climber's ropes) around the DB's so they're hanging at the bottom of the Range of motion for the exercise.
-S