Three simple ways to approach the problem
(a) use the original (and fairly simple equations) which were used to generate the majority of the P-charts. For the older charts there were two CASA-antecedent tech memos which gave the derivations from simplistic first principles. For some of the later examples which were produced prior to CASA opting out of local flight manuals, you might find charts based on the later (and much more rigorously complex) memo developed by JCF.
I don't have the books to hand but they are in my archives .. when I next spot them I will revisit this thread and put the older equations in for instance.
Alternatively, use some of the similarly derived equations from one of the standard references .. if I recall correctly the AGARD manuals had a reasonably useful set.
(b) do regressions on the charts as printed and generate a set of polynomials to suit
(c) similar to (b) but set up a set of look-up tables to do much the same sort of thing
Pretty simple non-rocket-science stuff ..