... don't know which "late gun writer" had an equation, so I can't answer your question as asked -- but --
The classic equation for twist (listed in Hatcher's Notebook, IIRC) is the one developed many years ago by Sir Alfred George Greenhill. It's helpful but a bit more restrictive than necessary (or to put it another way, it can stand loosening).
The classic Greenhill equation produces twists in terms of so many calibers or bullet diameters, and it uses a different numerical constant for each density of bullet material -- again, a bit tight for practical use.
Converted to produce twist rate in inches -- one turn in T inches -- here are the two versions of the Greenhill equation that I use as maximum and minimum twist. Note that both equations use the length of the bullet, not the weight:
T = twist in inches
d = diameter of bullet in decimal fraction of an inch (0.308, for example -- not 308)
l = length of longest bullet to be stabilized, in inches (this symbol is a lower-case L, not a 1)
maximum T = 200 (d�/l)
minimum T = 150 (d�/l)
Note also that the equation doesn't include velocity. A higher velocity requires a faster spin -- which it also provides.