What angle bend do you need the center of?
I believe the quick 'n dirty method would be to bend a scrap piece of pipe to whatever angle you need, then use a level or some other straightedge held against the adjacent sides of the angle to make a couple of straight lines forming an X inside the elbow. The center of the X will pretty closely be the center of the bend. Then you can just chuck the bent pipe back into the bender and transpose the center mark.
It's a lot easier to show a picture than to describe it, I'll see if I can conjure something up off the 'net and post it shortly.
There's probably a more mathematically precise way to do it, but I learned that trick as a 1st term apprentice working at a paper mill. Not all of the millbillies are keen on doing math problems at work