It's a simple matter of stop math.
You can use sunny-16 basics of having good exposure with shutter speed and iso at reciprocals (e.g., iso 100 and 1/100s) at the following apertures:
||Dark with sharp edges
||Soft around edges
Or, just take a regular shot with automated exposure settings the camera sets, and if it looks good, then do the math to see how many stops it's going to take you to f/1.4 AND however many stops you want to underexpose the ambient.
So, say, you're in sunny-16 conditions. That means, good exposure is at iso 100, f/16, and 1/100s.
If you're stuck at sync speed (1/200s), then you can only whack -1EV off that at base ISO, so the equivalent of bring the aperture to f/11.
Between f/1.4 and f/11 is +6EV.
So you need a -6EV (6-stop) neutral density filter if you're not underexposing the ambient at all. But if you also want to "pop" your subject by underexposing and lighting them more brightly than the background, you'll probably want 7EV or maybe even 8EV.
You can get a set of ND filters of different strengths that you can stack. Or a variable ND filter that uses two linear polarizers so that rotating one over the other adjusts the amount of filtering.
I’d like to be able to use this lens in the daytime along with a speed light with a softbox so that I can create portraits where the background/ambient exposure is darker, and then fill in my subject with my speed light.
Just me, but this may be wildly optimistic. To darken the background you need to light your subject more brightly than they are by the ambient light. How much darker the background gets depends on how much brighter you can light them.
Speedlights are like the f/3.5-5.6 kit lenses of the lighting world. They're limited in how much light they can pump out. And they're very good at focusing the light, but not so much spreading it out. Using a speedlight in a modifier, to "overpower" the noonday sun in sunny-16 conditions? It's a big ask, and it's typically why folks grab an AD200 Pro (200 Ws) or AD600 Pro (600 Ws) to do this instead of a ~70 Ws speedlight.
You can use a speedlight this way for fill, or as main illumination more in the golden or blue hours, or with your subject in the shade. But playing day for night with the noon day sun is more the province of monolights and pack and head systems, not so much speedlights.
Few last thoughts, while using an ND filter will not put you through the additional -2EV power suck of HSS, it's still going to darken and reduce both the flash and the ambient. And as an additional filter it can still introduce flare, particularly if you're shooting into the sun. And HSS means you don't need the right ND filter combination and that you can actually use a higher shutter speed for things like freezing action in the ambient. You might still want to consider getting a speedlight that can actually do HSS (like, say, a Godox TT600).