If you use the same reference point on both curves, 4 inch wider radius will clear just about anything you run with the exception maybe (I don't have any) of some big articulateds. I use 52 inch radius and my Max Grey 2-10-4 on the inner curve easily clears 80 ft passenger cars on the 56 inch radiius wider curve.
If measure from the middle on one line, measure from the middle on the next. Or a simple fix is to lay the tightest one, the 54 inch in this case, and then mark 4 inches out from (for example) the inside of the ties, and lay the inside of the ties on the next curve at those marks. You could make a mark every 6-12 inches around the curve to guide laying the flex track.
I don't understand what the purpose of the cross reference to sectional track 21.21 inches is for.
