Party effective level

[Jacob Marley]

Yes, it's an art, not a science, and a certain amount of estimating and adjusting is called for (read: "trial and error"). But you need a starting point, a base line to begin with.

If you are looking for a base line, I suggest using the iconic builds from Enemies & Allies - yes, I know it is a 3.0 book but it serves its purpose as a base line. Each iconic is built at levels 5, 10, & 15. The more optimized your party is in comparison, the greater the challenges they can face. Likewise, the more skilled your players are, the greater the challenges they can face. Be careful, though, that you do not adjust things too hard, too fast.

Alternatively, you could look at the various builds from the D&D Minis cards. You can find them here.