I have a number of systems but have TTL flash only on one system. On another system, I use a non-TTL flash (generic) flash in either auto or manual mode. Past experience tells me that using a generic flash in the auto mode is very cumbersome, what with the zoom, aperture and so on.
So I'm thinking if we could build an easier rule of thumb to use in the manual mode for daylight fill based on the guide number of the flash, similar to the sunny 16 for correct exposure on a sunny day. On a sunny day, the backlight or side light must have similar luminance (otherwise Sunny 16 wouldn't stand), and the fill to lift a shadow on the subject's face should be of similar output as well. So, for example, if you needed 50% of the output of a GN 20 flash to nicely lift shadow on a sunny day with the subject 5 meters away at f/4, for this exercise, the GN sufficient to lift shadow on a sunny day is 14. If I had a GN 28 flash, I can accordingly increase or decrease the flash compensation based on the distance and aperture. (So, for example, if the distance to subject is still 5 meters at f/2.8, I can knock down the flash level to 1/8.)
So the question is, what's the amount of light (in terms of GN) necessary to lift shadow on a sunny day at a given distance and given aperture? If we have that number, we can build our own rule of thumb for our respective flash. Has anyone tried to come up with this kind of rule of thumb for manual daylight fill?