I just started working myself through the Book "A Practical Guide to Lightcurve Photometry and Analysis" from Brian D. Warner.
There, he gives in many details, how to get absolute magnitudes from the desired objects. It is some more effort than the differential photometry described in the AAVSO CCD Manual.
Now my question: Why is the AAVSO method working so good? When I compare my (differential) measurements with those of others, they seem to be in very good agreement, although there have been no corrections for extinction, airmass and transform values.
Usually, it is said: if you want to compare measurements from different nights and different observers, you have to convert your differential magnitudes to the standard system. So you have to evaluate extinction, airmass and the transform values for your own system every night.
So when is it REALLY worth the effort for absolute photometry? Only when I want to study astroid lightcurves?