Quote Originally Posted by Stephen Benskin View Post

Finding log-H
log((1 / antilog Step tablet density) * Illuminance) = log of transmitted light or log-H
..............................
Also, note you can convert transmittance, density, illuminance etc to logs then you can add and subtract rather than divide and multiply. So the exposure to your film at the speed point is the sum of the log of step density, log of additional ND over the light and the log of the lamp's meter-candle-second (or lux-seconds)