ISO3200 film + ND filter question
I have a roll of 3200 film in my RB67 that I was using for some indoor shots. There are still a few frames left which I will have to shoot in sunlight. In order to get my shutter speed at or below 1/400 I am planning on using an ND filter (Cokin P154) which has a 3 stop effect. I figured that a 3200 speed film with a 3 stop ND filter would have the effect of acting like ISO400 so far as exposure settings are concerned.
To check this out I tested my digicam with and without the filter. At ISO400 it showed a shutter speed of 1/8 @ f/2.8. (I am indoors at night testing this out). At 3200 the shutter speed increased to 1/50 which is a 3 stop variance. I then put the 3 stop ND filter on the lens and at ISO3200 it gave me a shutter speed of 1/10 @ f/2.8 which is about 2 1/2 stops down in shutter speed from 1/50. However the results were close enough to validate my original theory.
Are there any other factors I should be taking into account when setting the exposure?