[Agemegos]

Originally Posted by Captain Joy

Could you clarify a bit? You’re asking for a “formula” for luminosity as a function of mass for low-mass stars?

I think the OP wants a formula for the luminosity of a star at the beginning of its life on the main sequence, as a function of mass. That's the value given as "L-Min" on the Stellar Evolution Table, p. 103 of GURPS Space.

In real astrophysics that is called the "zero-age main sequence luminosity". There's a fitted analytical function on the second page of Tout, C. A., Pols, O. R., Eggleton, P. P., & Han, Z. "Zero-age main-seqence radii and luminosities as analytic functions of mass and metallicity" Monthly Notices of the Royal Astronomical Society, Volume 281, Issue 1, pp. 257-262, but I don't think you're going to like it.