drk - significant nickel discovery - summary, page-9

The graph, as it says, pertains to a single orebody. Fair to say that the correlation is fairly universal for primary Thompson Belt type mineralisation, particularly where it has proven to be a Ni orebody. But the slope is not.

Starting at origin (0,0), going to the peak of the y axis (100% sulphides) the trend can be either very steep (~0,100)(nearly no Ni) to very shallow (34,100) (pure pentlandite)

Due to the variability in slope, there is no linear formula to guess how much Ni there is based on pyrrhotite content as you infer.