Using an iOS light meter app to figure compensation for ND filter

[Larry Wilson]

I'm not exactly a beginner, but I am a little foggy on this particular area:

I am planning to shoot Vision3 500T in daylight in both Super 8 and 16mm, and I am using a ND0.9 (ND8) filter as suggested by Kodak, as well as an 85B filter for daylight correction. The iOS light meter I'm using (Lux) has sections where you can set both the ISO of the film and/or the compensation, which I have set up for 1/3 stop increments.

My question is this:

I am setting the ISO to 320, as that would be the daylight speed for the stock, but I am unclear on setting compensation. Since the ND0.9 holds back three stops, do I set the compensation as +3 stops or -3 stops?

Also, if I just choose to set the ISO directly instead so I wouldn't have to deal with compensation, would that final ISO be 80 or 40? Again, I'm really bad with math ?

Any assistance would be most appreciated. 

[David Mullen ASC]

If you are starting at 320, then losing 1-stop is 160, 2-stops is 80, 3-stops is 40.

[Larry Wilson]

Thank you. That tells me everything I need. If I'm going to have to shoot at ISO 40 with V3 500T in daylight, maybe I might be better off keeping that as a nighttime/low light only stock and load up with V3 50D or maybe even 200T. Think some experimentation might be in order...

Looks like my math was actually pretty close. Thanks again.