Setting lighting ratio, remote 430 & 580?

I have the 2 flashes plus a set of skyports and a couple of umbrellas (one silver, one translucent). If I set up both lights at the same distance from my model and set them both to the same power level (1/2 power, manual) I'm assuming I still won't have the same light output because:

a. The 580 is more powerful than the 430.
b. More light is lost shooting through the translucent umbrella than shooting into the reflected umbrella.

It all has me wondering, short of using a flash meter, how do I set my flashes to be the same light output or dial in a 3:1 lighting ratio? Trial & error?

If my assumptions are correct, and I shoot the 580 (stronger) through the translucent umbrella (more loss) and shoot the 430 into the reflector umbrella (less loss) will I be lucky enough that the difference in losses will offset the flash unit output differences? Na, no way!

Your assumptions are correct, though not all umbrellas are equal in efficiencies.

With the flashes in manual mode, you can do some tests to predetermine what the flash/umb combo can provide. But without a light meter, I think the trial and error method will be faster. And the fact the EX speedlites can be adjusted to very low outputs, you shouldn't have problem getting the ratio regardless which umbrella to use.