Bruce Having a register to work from is good idea BUT Becareful it can bite you with round rail.!!. . . . The base edge of round rail is often not very straight (Or Flat) so carefully check that
1: It's straight
2: The distance from rail to Edge is consistant along it's length.

If it is straight and consistant then you can work from the register when setting rails parallel if not then work direct from the round rail.