Your not really "flushing" or 'finishing" on a final set. The thing is you probabley have got all the blood you will need from the former sets, if you worked them as you should, at that time. It's like placing a empty glass under a water faucet and turning on the water. Only so much water is needed to fill the glass. You can run the faucet for hours and no more water is getting into that glass. Same with muscle tissue, only so much blood is going to be received by the muscle. Flushing, finishing or pumping beyond that is pointless. It does not insure more growth. This can also apply to the drop set system.
Higher rep's seemed to be equated with lighter or moderate weights. Using heavier weight is always possible but higher focus will be required. Adapting a form of the rest-pause system (usually 15 seconds between reps) can handle this quite well for most guy's. Of course moderate weight works well when the intensidy is held high. If engaged in 15 to 20 rep set's, than the set's themselves should be kepts lower, as 2 or 3 sets each exercise. I'll do periodization, using a higher rep scheme, for 6-8 weeks with-in a training year.
If counting rep's becomes boring than try a different way of counting reps, rather than the usually 1-2-3-, etc to reach 20. Start with 1 but say 'and' for the next rep (really 2). For the rep after that say 2 than 'and' again, etc. AS: 1-and-2-and-3-and-4-and-5-and, etc, etc, etc,. When your near the end of the set of 20 you will be counting 10 (19) and the last rep will be "and" (20th). This type of count tricks (for a beter term) the mind into thinking your only doing half the rep for that, or any other set. Feel free to count on your fingers, I do. Good Luck.