I agree here. Here's how I'd determine it:
1) Set up a variety of encounters, say 4 of them. Make 'em around EL 20 (19-21). Try to vary the types wildly.
2) Create 5 PCs- say 1 rogue, 1 Fighter, 1 Cleric, 1 Monk, and 1 Wiz. All LV 20.
3) Try out the above encounters, 1 after another with the...