Is there any way to -approximately!- calculate how to adjust a correct aperture when you bounce the flash in a fully manual camera? With the newer models you don't need so, since the camera reads the amount of light that reaches the subject and stops the flash when that's enough. But with older, fully manual cameras, you only have what the guide number of the flash tells you, but that will only be accurate when you shot the flash directly to the subject. What happens when you bounce it? Any "rule" to more or less compensate the loss of light?