hi gerd.

i am kind of a luddite when it comes to these sorts of things but whenever i use a flash and bounce it ...
i need to add to the distance from the thing i am lighting when i figure out my fstop. for example ..
if the "thing" is 8 feet away, and i bounce to it, well it is xfeet from the ceiling and the ceiling is yfeet from me
s0 it will be x+y feet ...as others have mentioned the total amount of light doesn't make it to the subject, so you have to
take that into account too ...
sorry to suggest this, you might think of picking up a cheap flash meter ... years ago i picked up one and it makes it easy to
make sure my i am exposing things correctly ... some, even the cheap ones, allow for 3 readings, so you can do ambient readings
as well as flash readings at the same time ...

have fun!