Only way that the casual observer can even remotely come up with the answer is as per A Harman's approach

here.
"straight weights" are Newtonian, and their viscosity doesn't change with the rate of shear. Multigrades that have VII have two viscosities, the low shear, and the HTHS, the HTHS being lower than it should otherwise be, which is reflective of the polymers shearing.

A Harman's technique compares the calculated "KV150" with the HTHS, and gives them a percentage comparison.

Take the darling of the board.

TGMO, KV40 36.1, KV100 8.5, density 0.851...HTHS is quoted as being 2.6.

Gives a "stability" of 84.4

Compare it to Citgo 20W20

KV40 64.7, KV100 9.0, density 0.878, HTHS 2.9

Gives a "stability" of 97.5, which it should as it's a straight weight.

For a given grade, density and VI will give you a feel for where they get their KV values from, basestock or VII.