We seem to have some confusion over "f/4.5". It normally means "aperture is focal length divided by 4.5", which is 1/3 of a stop smaller than f/4. 1/3 again is f/5 and then a third again is f/5.6 - the next whole stop. However on a Minolta flash meter if it says "f/4 5" where the 5 is a little digit just to the right of the f/4, that doesn't mean f/4.5. It means "f/4 plus half a stop". Similarly, "f/4 9" means "f/4 plus 0.9 of a stop" (very very close to f/5.6) not f/4.9.
So if your flash meter reads f/4+0.5 and you take 3 stops off, you get f/1.4+0.5 which is about f/1.7 on the aperture dial - halfway between f/1.4 and f/2 on a mechanical aperture ring
With a stationary subject you can certainly put the camera on a tripod, focus and then add the filter - it's what I do when shooting with infrared filters. But it's a real hassle unless you're using a TLR (look through one lens, shoot through the other-with-filter) and it's not feasible for even the most slightly-moving subject or when shooting handheld. Since you're shooting indoors, focusing critically wide open is difficult anyway; doing it handheld with an ND8 on there is crazy-talk.
Edit: if there's no human in the shot, you could just light it with a lightbulb and use a longer exposure; no need for flash. Or you could use a smaller (battery powered) flash. It's really useful to have a couple speedlights on-hand because the good ones go to really-low power, they're easy to move around (no power cord!) and they're cheap. They make good accent & rim-lights if nothing else.