f/1.8 is 3 stops less than f/5, so bump your flash up 1/3 of a stop (or move it a smidgen closer) so that you get f/5 on the meter (also known as f/4+6 on most flash meters).
So your flash (without filter) measures f/5 at the subject at minimum power? If you put the ND8 on there, you will get enough light for f/1.8 - you can see that for yourself by metering through the filter. If you want to shoot at f/1.8 then just go ahead and do so by leaving the flash on min power and putting the filter on.
You're correct that shutter is irrelevant to all of this except for balancing ambient. However with large apertures like this, you are likely to get some ambient contribution so you might actually need to pay attention to the shutter speed - especially true if you've got natural light coming into your scene at any point. If you put the ND on the camera, you will cut down the ambient very nicely but it's quite hard to focus accurately. The other alternative is to put the ND on the flash - focusing is easy, but obviously it means you have 8x more ambient light coming in.