In the latest issue of Acta Acoustica united with Acustica, Eddy Gerritsen (TNO) discusses engineering methods for the prediction of reverberation time and sound pressure levels in spaces with non-diffuse conditions. The assumptions and theoretical considerations behind the formulas in EN 12354-6 annex D are clarified.
There is definitely a need for engineering models to estimate reverberation time and sound levels in this large group of rooms where the acoustical treatment consist of an absorbent ceiling. The discrepancy between Sabine calculations and measurements in e.g. classrooms, offices, day-care centres among others, are well documented. So, if you are interested in this matter I recommend you to read the article.
The abstract is available here.