If the version you use allows you to set the curtain, then
just look for the point in the raw graph, un-treated, for where your room response starts to smooth out on its own. I believe the smaller the room, the higher the frequency, but usually between 200-300hz.
I am not a Dirac user, nor do I use PEQ, so your other question is pushing outside my zone of comfort. Generally speaking, I would use room correction first, then adjust afterword. Dirac, as I understand, gives you better controls of how it works/what it does, than say Audyssey will. Depending on the version/level of Dirac, I think you can fix anything using their program, at least based on what I'm learning from listening in on some other conversations.
Beyond that, you will need to talk to somebody much more familiar with Dirac.
Cheers!