I am assuming that you cannot turn the strobes down anymore then what they already are? OK, so you are measuring at F4.5 (@ your desired ISO, of course), then adding a 3 stop ND8 will mean that you will need 3 stops more light to hit the film.
SO, you have 2 choices
1) Adjust your lights up so you have 3 stops more power
2) Open the lens up so you have 3 stops more light
So, since you want to shoot @ F1.8 (kind of close enough), you wouldn't touch the lights.
That being said, that if this was in the studio, if I had a digital on hand, I would be using that to preview what you are doing.