In which case should I calculate the provisional requirements after the ETM (rather than after the provisional classes)?

It all depends on how much information you have at the time of calculation. The calculation based on the ETM population is less accurate but can be done earlier in the year with less information. Note however that detailing the population at the level of the provisional classes is necessary to distribute the students and generate the courses from Pre-incoming year: If you have calculated your requirements and then developed the DTMD based on the ETM population, there is nothing to prevent you from specifying the provisional requirements per class in a second step. For example, once you have gathered all the necessary information at the end of the year.