Larry, after reading a bunch of the trailer conversion threads here, I went with 11" on center spacing for my PA12 (before I picked it up - wanted to be ready!)... Turns out, when the boat went on, it didn't quite work out that way...
Islandspeed has the best idea, sit the boat on there with the PVC under it, and mark it from there. Mine ended up with just under 11" oc at the front end, and almost 12" oc at the back... Mini wedge, sorta... I have a carriage built to fit into my mesh-side utility trailer... The boat overhangs front and back by about 4 feet, and the PVC cuts that down to two-ish on each end.
Good luck, have fun!