I started out doing the EL=CR for the encounter. It works pretty well. My lot were all about 3rd or 4th by the time they left Region A.
After A, I guestimated that it would take about the same number of sessions for the next region and started awarding xp based on that. If they go to B, say, I guessed that it would take 12 sessions to finish the region. To go from 3rd level to 6th level takes 16000 xp, so, I gave out 1500 xp per session. To my surprise, this was almost exactly the same as if I had been doing xp by the book. Since it worked, I do the same thing for each region now. If they are a little low at the end of a region, I give a region bonus to bring them up and if they are a bit high, which a couple of my survivors are now, I put a lid on the xp to let the others catch up.
Don't be too fussed about it.