You have discovered the wonderful, but complex, world of mixed lighting: congratulations!
Quick example using the Sunny 16 rule …
- Start parameters (suitable environmental exposure): ISO100, f / 16, 1/125
- Adjust to open for bokeh: ISO100, f / 1.4, 1/16000
Now, obviously, these new configurations have entered theoretical territory with that shutter speed value. Then, add a little ND and let's download it. If you want to have a darker / more flash style of the photo, then you will have to underexpose it with a stop or two (or three). Let's take two and redo our initial calculation …
- Start parameters (underexposed environment): ISO100, f / 16, 1/500
- Adjust to open for bokeh: ISO100, f / 1.4, 1/64000
Still in theoretical territory. But, let's add a bit of ND so that you can lower your maximum flash sync speed again:
- Adjust with ND: ISO100, f / 1.4, 1/125, 9stops ND
Then, there you have it. If the proper exposure was a sunny day of 16, and if I wanted the environment to be two underexposed stops, then I would need 9 ND stops and a very powerful flash. In fact, I would bet that a speed light will not have the juice to overcome the 9 stops of ND, but you can ask about that calculation in another question, if you wish.
However, if your initial exposure changes, so does your ND need. For this reason, trying to really calculate the filter 1 ND to govern them all is, in the best of cases, an educational exercise. Actually, you must carry several filters (3 stops, 6 stops, 9 stops) [thus allowing for combos ranging from 3 to 18 stops]) and / or a variable neutral density filter.