Here's how I do this:

1) set up the shot and the you will estimate the Flash to subject distance (let us assume 15 feet).

2) Calculate the aperture: f=GN/Distance(feet). Assuming I am using my Olympus T-32 Flash that has a Guide Number of 104 (feet) or 32 (meters) at ISO 100 the math is as follows:

f= 104 (the guide number for full power in feet)/15 feet (the distance from my flash to the subject) or; f=104/15=7

The aperture calculates to f7 at ISO 100 for 15 feet. If I am using 400 speed film, I stop down 2 additional stops. In this case I will use f16 or put a 2 stop ND filter on the flash.