You've discovered the wonderful, yet complex, world of mixed lighting - congrats!
Quick example using Sunny 16 rule...
- Starting Parameters (proper ambient exposure): ISO100, f/16, 1/125
- Adjusting to open up for bokeh: ISO100, f/1.4, 1/16000
Now, obviously those new settings have gone into theoretical territory with that shutter speed value. So, let's add in some ND and bring it back down. If you are wanting to have a darker ambient / more flash style of photo, then you're going to have to underexpose by a stop or two (or three). Let's assume two and redo our starting calculation...
- Starting Parameters (underexposed ambient): ISO100, f/16, 1/500
- Adjusting to open up for bokeh: ISO100, f/1.4, 1/64000
Still in theoretical territory. But, let's add some ND to get you back down under your maximum flash sync speed:
- Adjusting with ND: ISO100, f/1.4, 1/125, 9stops ND
So, there you have it. If proper exposure were a Sunny 16 kind of day, and if you wanted ambient to be two stops underexposed, then you'd need 9 stops of ND and one very powerful flash. In fact, I'd actually bet that a speed light won't have the juice to overcome 9 stops of ND - but you can ask about that calculation in another question, if you are so inclined.
If your starting exposure changes, however, then so too does your ND need. For this reason, trying to actually calculate the 1 ND filter to rule them all is an educational exercise at best. In reality, you should either carry multiple filters (3 stop, 6 stop, 9 stop [thus allowing for combos ranging from 3 to 18 stops]) and/or a variable neutral density filter.