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Instruction manual for a camera can't tell you what exposure to take flats with a light box, as it depends on how bright the light box is. What you're after is a histogram that isn't clipped at either end, and a peak in the histogram around 1/3 to 1/2 way along from the dark end. Depends who you talk to. I don't think the correct term is FWHM either, that's usually referring to stars. Maybe ADU count? Any way, if you use the histogram it's irrelevant as the peak will do the job for you.
My flat exposures are very short, especially for Lum filters. 3nm SII is much longer obviously. Don't worry about how short the exposure is, as long as it's not too fast for your shutter - ie you don't get half the image dark. If you're still getting even illumination, that's all that's important.
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