The bolt circle can vary, best method is to machine the backplate to a snug fit on the register, then make a set of three transfer punches- bushes to fit the bolt holes in the spindle flange with flanges to locate against the spindle face with 60-degree points (look like umbrellas with fat handles), short enough to slide the backplate on the.register, then a tap with a mallet will give three neat punch-marks on the bolt centres.
Or you could put pins in the holes, measure one pin with a digital caliper *then zero it without closing up* then measuring across two pins will give the spacing, jommetry [1] will give the circle radius/diameter.
Transfer punches are easier, though, and you'll have them next time you need to make a backplate...
[1] spacing/2 = r sin(60), rearranged gives
r = Spacing/2 sin(60)
r = spacing/root(3)
Just like star-delta three-phase...
