I have various old flashes of which I wanted to verify the guide numbers. For instance, I have two Olympus flashes for the XA cameras -- the A11 and A16 (those are also the guide numbers in meters at ISO 100). So I set my light meter to ISO 100 and fired the flash at it (at full power). Both flashes registered one stop over what the guide number suggests. For example, if the guide number suggests f/11, the meter says f/8. I tried it with an accessory flash (Sunpak 383) and it's the same thing. At least it's consistent, but I have to doubt that all the flashes are off. The meter is a Sekonic L-308B. I have the dome over the sensor, and I'm shooting from one meter away. The meter is dead on, in my experience, for my large format portrait work -- even with the IP instant film I'm using that has very little latitude. So I'm not sure it's the meter either. Everything has fresh batteries. Is my methodology flawed?