Neil, fix a nice plate to both bearings on your choice of master rail, then pop a clock dial on via a mag base featuring a long reach arm/boom to get to the other rail with the clock, then traverse the master rail via the plate and bearings dialing in your second rail as you go.

Ideally you would want to do this with out the bearings and plate and so just moving the mag base along the master.

That's how I always do it for the machines I build from wood .

.Me