Okay. Sorry, it took a while to locate a readable version of your document.
There doesn't appear to be any equation that develops these graphs which was what I was hoping for because that's what Mathcad does best. So we're stuck having to "brute force" this. I used WebPlotDigitizer(TM) to pick end points from the three lines in your plot. (I may not have been terribly accurate.) The attached file shows those values and recreates the plot in Mathcad (Prime 6.0).
Since the graph is a log-log, I suggest you do linear interpolation of the logs, I put in one equation (for a/R = 0.04), my Express won't run linterp, but you should get the correct value in full Prime.
It's not pretty, but it's one way. This would give you three points on a S vs a/R plot so you could interpolate that curve (suggest polyfit?)
Good luck
