If you process some film like Ilford Delta 100 to ISO conditions then the exposure required to get a 0.1 density on the film is 8 millilux seconds (by definition). If you take the log of this (0.9) then you can simply add in the density of your step wedge at that point (like 2.8 log d or whatever that step is) and that gives you the output of your sensitometer light. In the example it would be 2.8 plus 0.9 = 3.7. If you take the antilog of that you are back to millilux seconds which would be 5011 millilux seconds.