so the total of the DPs spent by a whole party should give a general idea of their "power level"
Now, I think you've hit it.
I have difficult encounters to be the same lvl as the party. Sometimes I need encounters to have specific scaling but there is always the posibility of TPK. Heck, you could almost say, "it's their right as players" to cash out.
Then I ask, 'If they overcome said threat, have I unduly biased the next encounter in the party's favor?' In other words, don't give enemies stuff the party can easily use. Players are sneaky about using this stuff too.
So they enemies snuck up on the group without a sound... that's because they're wearing 'boots of silence'.
Great! the party thinks, now the Dwarf and Humans can stop putting Ranks in Stalking.. Then you descibe the boots as the skinned feet of halfling sherrifs from the last town back...
Anyway... when the enemies are as tough as the party.. the scalable question might be "which group is smarter".