8 Che, all correct what you said, but when you are on the plane. Mentioned regulations are for the planning stage.
As you said, actual driftdown altitude for actual conditions are the matter of pilot, but net ceiling info is not the matter of pilot and useless info at this stage for pilot. Therefore AIRBUS removed related info from in- flight operation documents. For the actual conditions I also do what you do. However Net ceiling and the factors whch are deteriorating it (e.g CDL) is the matter of the planner. From planner perspective, Gross ceiling or driftdown altitude is not an issue. And the net ceiling is tried to be calculated according to best available weight and ambient conditions predictions. Planner considers if actual (e.g gross ceiling) is different than planned that is is pilot's problem. The amount of standard deviation between planned and actual shows the quality of operation of that operator; bigger deviation equals to worse operation quality.