Magic Aura Zones

Say you build rooms within a magic aura, and then certain areas experience more magical activity than others (beneficial botches, vis study, ritual casting, etc). Is there a standard method for just that area within the larger aura getting the magical aura variation roll from the activity rather than the entire aura, or is this another one of those YSMV things?

The "care and feeding" of Magic Auras is, in canon, Stark Naked Magic. There is no standard method. There is not even a hint of guidelines. According to the fluff, several have died trying to manipulate them. Pretty much all you have is ST fiat, unless you learn Hermetic Architecture.

By RoP:M vis extraction, ritual casting etc. can temporarily weaken or strengthen whole Auras (only). They do not modify Aura strength just in the small portion of the Aura where they take place (which would be hard to delimit precisely in any case). That's the simplest solution, and the RAW.

That said, there are several places described in the books, I seem to recall, where a stronger localized (magic) "Aura patch" has sprung up allegedely because of magical activities in a small region of a preexisting (magic) Aura. So, if your covenant lies within a gigantic forest, 100 miles to a side, all covered in a gigantic +2 magic Aura, it's perfectly reasonable to rule that long-term magical activities only modify the Aura, say, on covenant grounds.