I am not near desynthesis cap. I have two classes in the upper eighties and nineties, and a smattering in other classes. So, I have no experience as to what happens when you try to push a craft over the 300 limit. So, I'd like to ask, what precisely happens?

It was my belief that, when you are at 300, if you earn 0.5 in a craft, it will subtract 0.5 from another craft, chosen at random. I know already that you can protect a craft from being randomly selected by doing a desynth in that craft, and that will protect you from four desynths (for example, desynth a fish, and Culinarian is protected for the next four synths in other classes).

So, even if your desynth levels are spread across a bunch of different crafts, just protect the ones you want to keep, and the 0.5 should be removed from crafts you don't care about. If that is what happens, I really don't see the point at all in having a book to reset your craft. If you don't care about the points you have in a craft, just level up the ones you do care about and those points will eventually disappear... Is that not how it works?