Carjo wrote in post #5346446
Turns out I was using the metre-based guide number.

The correct number is 43.
That in mind, I get the following:
f = (g/d) @ iso 100
f = (43/2)
f = ~22 (or f/11 @ 200 iso)
I'm bringing all this up after the fact. I was mucking around with my camera yesterday while my friend and I were waiting for her bus, and f/11 was working better than anything wider.

Ok... next question: how do I determine the shutter speed to go with it w/o using a grey card? Trial and error?
Well it depends on how much ambient you want. If you don't care, shoot at max sync speed (1/200 on most Canons, although some do 1/250). If you want some background, drag the shutter more.
Other thing to watch is the GN calc only works without modification when the flash is on the same axis as the camera. If you're flashing from off-camera the GN calc is reduced, so if your flash is at 45 degrees to your subject then the GN calc drops by about 70%.
Chapter 10 in Light Science and Magic covers all this quite well 