• This is the procedure I usually follow when bouncing flashlight off the ceiling.

1. Focus at the bouncing point on the ceiling to measure the 'flash to bouncing point' distance. (i.e. 1.2m)
2. Stoop a little to be the same height as the photographing subject.
3. Focus at the bouncing point again to measure the 'bouncing point to subject' distance. (i.e. 1.6m)
4. Now I get the total length of the path between the flash and the subject (1+3) (i.e. 2.8m)
5. Estimate the reflective ratio of the ceiling. (i.e. 25% : 0.5stops blighter than the 18% reflective card)
6. Aperture calculating

(example)

film ISO100
flash GN 28m (ISO100)
total travel of light: 2.8m
-> 28/2.8 = F10 (provided that the bouncing surface is 100% reflective)
estimated reflevtive ratio: 25% (2 stops of light loss)
-> F5.0
safety margin (only for print films): half a stop
-> F4.0 (2.5 stops from F10 in total)

This procedure is only applicable when the bouncing point is located just between the flash and the subject (the incident and reflective angles should be the same), and the ceiling should not be too low. but it does work well for that use. You may feel somewhat apprehensive to estimate the reflective ratio, but most people usually have good experiece of estimating the ratio already. If the color of the bouncing surface is as blight as the 18% grey card, that means 18% of your flashlight will reach the subject, which is equivalent to 2.5stops of a light loss. What you have to do is compensating the aperture from there. And this is not so different from compensating your exposure setting when shooting under the available light depending on the in-camera reflective light meter.