I'll use this opportunity to get back on my desire soapbox. Desires are what we actually work with; pleasure and pain are reactions. I've got a working hypothesis that the categories of desires describe the upper and lower limits of desires (not pleasures), and that by defining these for oneself, a person develops a personal sweet spot for working with desires in order to discover what leads to their most pleasant life.
In the Principle Doctrines, it seems that the Doctrines on pleasure go from more theoretical to more practical as you progress through the Doctrines. So I'm also thinking that PD18-21 may actually be more pertinent, once one has worked through the earlier ones. I think that they build on each other. After that come the PDs on desires. One's understanding has to progress through the sequence in order to gain a practical understanding of the ethics.