The idea of using a flash isn't very different than using a EG&G Sensitometer. The output is alot higher than is needed so ND filters are usually required. If you want to measure the output with an exposure meter, I would measure the flash without the ND filters and then caluclate them in. Not only would it be difficult to measure something like 1.5 lxs with a photographic exposure meter, I would question the meter's accuracy at such a low light level. The meter is probably more accurate at normal pictorial levels. I did some research into finding a light meter that I could use to calibrate my sensitometer and the price was around $4000.00.
Rafal, I didn't go over everything, but so far it all looks good. There is one small thing though. Luminous exposure is H, and I believe the 99.25 lxs value would still be considered E because it has yet to pass through the step tablet. H being the exposure at the film plane.