Today was round three of my flash testing. I set up a tripod and used a shutter release. I tested metering mode, effect of large white object, focal point, ambient light level, ISO and flash exposure compensation (FEC). Nothing made much of a difference except the FEC (obviously). As a tentative rule of thumb I concluded that for every stop underexposed add 2/3 FEC. The thing about this experiment was that all of the view of the photo was the same distance from the camera. I decided one more try in the morning using a scene where there was a lot of range in the scene.