With some help from Kevo, we started at the bow end and cut the rail to fit. Beforehand, I had cut test pieces on gunwale scrap at both the bow and transom and recorded those bevels and angles.
Somewhat scary as it's loaded pretty tight, pretty much like an archery longbow. Something slips and it could be a pretty stout whack to the face. Or worse, the rail could break.
Note that Brad does this a different way, clamping pre-cut templates to each end and then measuring the gap in between. I didn't have enough left over gunwale material to allow the gap to be narrow enough to keep the measurement, which is on a curve, accurate.
Things got better with practice and eventually determined the best method, at least for me, was to mark a line-up spot on the rail and oarlock spacer and start in the middle and work out, adjusting each line a smidge one way or the other to final fit each end.
After dry fitting, the rail was epoxied and clamped.
Enough for one night.