ASTM D2270 Viscosity Index Formula/Table derivation How was the formula (3) for viscosity index $\mathrm{VI}$ derived in section 5. Procedure A—Oils of Viscosity Index Up to and Including 100 [1]?
Specifically with reference to the reference oils considered and the table for it.

$$\mathrm{VI} = \frac{L - U}{L - H} \times 100$$

References


D02 Committee. ASTM D2270 - 04 Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100 °C; ASTM International. DOI: 10.1520/D2270-04. (PDF)
back when it was invented they measured all the oils they could find.  They got the worst (most viscosity change) and the best in each group and ranked these as 0 to 100.  All others were plotted on this "linear" scale.

The unit was used for dome time until the best was bettered, then they invented VIE (E is a subscript, and stands for extension, strictly all VI's above 100 are VIE's).

It is a horrid unit because it is far from having any useful meaning, unfortunately it has some significance.