ND Calculation Help - ISO 3 & Flash

Dear LF world -

Quick question, would just like some help to confirm the below..

Project-
Shooting 8x10 Paper Negs Portraits, 600mm f9 @ ISO 3, with Flash.

However, would like to preview on digital before hand, WITHOUT altering flash strength.

At ISO 100 on the digital, I think I require a 5 Stop ND filter to make it the equivalent of ISO 3?
Is that correct??

Then lets say, due to bellows compensation factor, I need an extra stop exposure, I would then need an extra ND stop? Making the equiv ISO 1.6?

Any comments would be much appreciated!

Thanks!
Matt

Count on your fingers. 100, 50, 25, 12.5, 6.25, 3.125, 1.5626, ... 0, -1, -2, -3, -4, -5, -6

5 stops from ISO100 to ISO3(100 50 25 12.5 6 3)is confirmed.