I think sometimes there's too much of an urgency to find a consistent, mathematical answer that can be applied evenly, across the board to solve a problem. This often isn't necessary.
A trick I've used DMing across all editions; if the battle is "over" and grind sets in, and I can't think of a good reason to just have the monsters run away, I just turn those monsters essentially into minions. The next attack that hits them kills them.
No need to run every monster's HP through a calculator to figure out what 80% of their base is; no need to pump up damage or reduce defenses (well, maybe if you're using MM1 monsters, but that's another discussion entirely); just an easy fudge that gives you the power to end the combat essentially when you feel it's time to end it.
If you find this gives the players too much longevity, just add in more quick skill challenges, traps and hazards to soak up a couple extra healing surges. The only difference your players will notice is that combats always seem to end right before they get too grindy.