I'm familiar with the maths behind flash usage (guide numbers, etc.). Here's a more practical question that doesn't yet seem to have an answer on photo.se:
Consider a subject, outdoors, on a regular "sunny 16" day (EV 15). Assume you've rated your film @100. For some reason, you want to seperate your subject from the background by underexposing the latter by, say, two stops. Say, f/11 & 1/800 rather than f/16 & 1/100. Now in order to compensate for that underexposure, you want to use a flash on your subject to get it back to EV 15.
What guide number would the flash have to have for that purpose?
GN = d * f
If the subject were at a distance of 2.5m, a GN 40 flash would provide enough light for a f/16 & 1/100 exposure (in a setting that's completely dark otherwise). So using that flash on our subject would exactly double the amount of light it receives. Am I correct assuming that this in turn means we've been able to compensate for one of the two stops (EV 14), but won't be able to compensate for the remaining one?