Testing old film speed with Flash?

I have some old bulk loaders with film in them. I just did a "clip test" and found out they are Old Tri-x 400 and Old TMax 100. To test what speed I should shoot them, is it OK to do that with flash?

I was thinking of setting everything up and just start varying the aperture to see what a good exposure is. Is this alright to do, or just use ambient light outside?

I'd use the same type of light I was planning to shoot in. For me that would be ambient.

sense

That makes sense to me.

I plan on shooting both types of lighting (and a mix), so I'll try both.

Thanks.