I'll have to look. Kolbe gives Powley's equation without so many pre-figured terms as Davis uses as:
Where:
L = Powder Weight in Grains
G = Bullet Weight in Grains
r = Expansion Ratio
Looking at the lower form, a subtraction is involved. So we'll have to look at Davis's terms I, N, and Y to see if they match up?
That equation reveals one of the limitations of the Powley's simplified approach. Powder weight in a velocity equation only works if all the powders you are using have the same energy content per grain. The IMR single-base rifle powders are certainly close to one another, but there are other powders which are not.
You also have to assume the burning gets pretty well done by bullet exit from the barrel. Kolbe has a graph of measured velocity verses that calculated by the Powley equation. For a .30-06 load in the example, they don't agree well except at zero and from about 16" of barrel forward. For a slower powder, presumably the barrel has to be still longer before agreement is good. 5% seems to be the equation's accuracy limit, but Kolbe points out that all the other factors of barrel friction, chamber size, primer choice and so on, can change velocity 5% anyway. So, he feels the equation is good enough at 5% precision for application to an unknown gun.