I use a formula, and this assumes a 5-man rotation:
TPC = (1 + ((DUR-20)/20)) * ST, rounded down; MPC = [formula result] + 5. I basically always use this during the regular season for pitchers with DUR < 26.
For pitchers with DUR > 27, I use the formula for MPC, but for TPC, I choose something between (1.3 * ST) and the formula result, and I give better quality pitchers a higher TPC. The rationale for doing something different with high DUR pitchers is that they can tolerate a higher level of post-game fatigue and still recover in time for their next start, but in-game fatigue may still render them effective deep in games. My aces may continue to be better than my bullpen with a little more in-game fatigue than my 5th starters, so I target them to go longer.
For a 4-man rotation I just multiply all TPCs by 0.8, and I don't ever mess with the formula unless DUR > 33 (and that high a DUR is uncommon for a pitcher with high stamina).
And in the playoffs I set TPC and MPC game by game.