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...I almost exclusively shoot in aperture priority, so I'm afraid this might not work for me?
Since shutter speed (as long as it's within sync speed for your camera) doesn't affect flash, just set the shutter speed on the meter for your sync speed and take a test pop. If the displayed aperture isn't the one you want, then adjust the power of the flash the number of stops needed to get the desired aperture.
For example, let's say you want your fill light to be 2 stops below your key light, and you're shooting at f/5.6. If you meter the key and it shows f/16 you'll need to dial it down 3 stops. If you then meter your fill and it also reads f/16, you'll need to dial it down 3 stops to f/5.6 plus 2 more to get your desired ratio.
Once you've done it a few times it'll be fast and easy.
(BTW, in case you didn't understand what I meant by "shutter speed doesn't affect flash" -- whether you have your shutter speed set for 1/250 or 1/60 or any other speed at or below sync, the flash pulse is only going to be 1/800 or 1/1600 or whatever your brand of strobe will be, so 100% of the light will reach the shutter regardless of the shutter speed. If you go above sync speed, then only a moving sliver of the sensor is exposed at a time, so flash won't (in general) work.)