Guide Number arithmetic only applies to direct flash...a Guide Number is flash-to-subject distance ordinarily, but by bouncing that calculation is useless.
The ceiling-to-subject distance might seem to have a factor, but when a flash is bounced up to the ceiling, the ceiling is a large virtual source which is NOT governed by the Inverse Square Rule of small source falloff in intensity.
To illustrate, I just did this with my Metz 54MZ, which has GN106 with 'normal lens' coverage angle. With my subject exactly 7' from where I stand, GN arithmetic says f/15.
Most every speedlight is overrated by the manufacturer (virtually every brand is guilty of this!), and most will actually flash meter LOWER than rated by about -1EV! Direct flash from Metz 54MZ standing 7' away indicates f/11 +0.0EV with Minolta flash meter (GN77) vs. manufacturer rated aperture says f/15 (GN106)
Standing 7' away from subject, and using ceiling bounce which is 7' above the subject (subject to center of bounce) meters f/4 + 0.6EV, or GN35.
Measured direct flash = GN77, while ceiling bounce measured GN35...about -2 + 0.3 EV loss due to bounce from measured GN for direct flash.