I'm not exactly sure what you are looking for. There's the way to determine what the log-H is with a step tablet, and there's the classic camera exposure equation. There is also "P" which is the midpoint exposure that the meter aims to make. P = 8 / ISO

Finding log-H
log((1 / antilog Step tablet density) * Illuminance) = log of transmitted light or log-H

Finding Transmitted Light
Take the speed you have assigned to the film and plug it into:
0.80 / film speed = transmitted light necessary to create the 0.10 over Fb+f density
example 0.80 / 125 = 0.0064 mcs

Finding the Incident Light
Incident light = transmitted light / transmittance
Find the step tablet density that created a film density of 0.10 over film base plus fog
Take that step tablet density along with the transmitted light value and plug it into the equation:

Incident light = transmitted light / transmittance
example: 0.0064 / (1/10^2.75) = 3.6 mcs (lxs)

Calculating Exposure

From "Is the K factor relevant to me or should I cancel it out?" thread.

Defining K, part 1.jpg