The Ross O72 curve components are set at 22.5° of arc, except for the through of the O72/O54 curved switch which is about 26. 6° of arc. The custom curve should be close to 18.4° of arc - cut out of the 22.5° factory curve.
Measuring a straight line end to end on the outside rail would be ~ 11.75" long, give or take a couple of tenths.