I required one information regarding the extraction of stiffness coefficient value from fig of appendix P API650.
My question is that if we have the vale of a/R other than define in fig i-e a/R=0.005, a/R=0.02, & a/R= 0.04 then is there any method to locate the a/R line(e.g as per calculation it becomes 0.0025) in that figure? Defiantly these three lines must have some basis for their locations in the fig., but what are that basis?

The references used to formulate Appendix P are given in Appendix P. The information you are concerned with was obtained from the instrumentation and testing of two full-scale tanks, with subsequent FEA verification.

When you're between the lines, or Figures, interpolate.