Is it possible to know the equations (as in coordinate geometry) which correspond to the FERRITE NUMBER (FN) lines on the WRC 1998 graph? OR are empirical data for Nickel equivalent and Chromium equivalent available for showing how FN lines have been derived?

I would like to know the answer to this question on material published by ASME Boiler and Pressure Vessel committee Subcommittee on materials in Vol II Part C Specifications for welding rods electrodes and filler metals.

In page 93 of edition dt. July 1, 2004 there is figure A3 WRC 1998 dealing with FN (Ferrite Number) diagram for Stainless Steel and weld metal.

Could I please be directed to a source from where I might get the answer.

THANKS.