For instance, if a grain is specified at 300 SRM, what is the equation to convert that to Lovibond? What is the equation to convert it back?
First, a little history. Joseph Lovibond developed the Lovibond Scale in the 1860's, as a means of implementing quality control into beer production. The Lovibond Scale works by having the user visually compare the shade of a substance to the nearest shade of colored disks. Thus, determination of a color on the Lovibond Scale is a human estimation, rather than a scientific measurement.
SRM, or standard reference method, is a more recent method, which measures the absorption of a substance using a spectrophotometer. The SRM was developed as an answer to the inaccuracies found in implementation of the Lovibond Scale.
The variance in measuring the color in °L has led different beer experts to develop different methods for converting between the two. They are as follows:
where W = weight of malt (in lbs.) L = color of malt (in °L) V = volume of wort (in gal.)
Morey's formula gives a better approximation than the others, since it is a nonlinear conversion. However, if you want simpler math, both Mosher and Daniels' methods are good approximations.
Beer color is usually measured in SRM. Malts are often given in °L. In practicality, the discrepancy makes sense. I've never seen the inner workings at a maltster, but I imagine color is determined by holding a sample of grain next to a color palette and estimating it's darkness by comparison. On the other hand, if someone wanted to know the darkness of beer, it wouldn't be difficult to run a specimen through a spectrophotometer, and measure it's absorbency at 430 nm. It wouldn't be possible to run the malts through a spectrophotometer to find the SRM value, so °L is given instead, and human error is deemed acceptable.
From what I understand, the formula is:
°L = (SRM + 0.6) / 1.35
However, for all values light enough to be visibly different to the human eye, L=SRM. They only start splitting when you get too dark to be visibly different without a spectrometer.