Yes, the flash won't go down further than that. I think I've done my calculating right, going from f4.5, to f1.8 will be roughly 3 stops. But just to check that I'm working it out right: say my scene is metering at f11 instead, and put the filter on, I'm going to need to come down to f4. Yeah?
It is in the studio and there is a digital on hand. Definitely will check it first before I sacrifice precious film! I have Pola too.
Would there be much of a difference in the image if you were to pump the flash output up rather than open up the lens?
It just comes down to giving the film the right amount of light - the method shouldn't matter...?