Flashmeter or testing are the options. It's not that big a deal to test, if you don't have a flashmeter. Before I had a flashmeter, I tested all my studio reflectors and diffusers and came up with guide numbers, and would map out studio setups in advance before shooting, so I would have the right f:stop. As long as I didn't change the light-to-subject distance, I had some flexibility in moving the lights during the session. It was absolutely reliable.

So do a test with slide film at a non-macro distance so you don't have to calculate exposure factor. Figure a softbox is going to cost you between 1.5 and 2.5 stops and shoot a series of frames a half stop apart.

Then when you are shooting macro, don't forget to include the exposure factor for high magnification.