Published guide numbers are never right, you must create your own based on actually taking pics. The procedure is simple, Set the flash 10 ft from a target and make a series of exposures, on any film, at one stop intervals. Look at the results and pick the one you like and multiply that stop by 10 and you have a usable accurate personal guide number based on your equipment and technique. A couple helpful notes-put the stop on a card in the scene and make it large. Putting the stops (2.8, thru 11 or more) around the edge lets you put a clamp (clothes pin) pointing to the stop without having to setup cards for each exposure. Whatever you do don't use a Macbeth chart if you don't have a transmission densitomer to read the film...they're worthless visually. If the info is on the film it's more helpful.