How exactly do you know where to put a fin?
Personal preference, based on years of knowledge (or guessing) based on tail width, fin size, waves encountered.
Most general, place leading edge close to trailing edge of backfoot IF you want quick turns.
On multi fins, you gotta figure the general center of effort of all the fins combined, then think about original intent of the fin cluster design, and make placement accordingly.
Experience, plain and simple. You can be told where to put a fin… but to understand it takes a lot of trial and error. -Carl
Umm… lots of trolling through the archives 
But seriously, there is so much good experience on here that research in the archives and matching it with your preferences should get you what you are after.
-Cam