I go by the size of the center hub. The larger the center hub, the further away the fan needs to be to eliminate the deap spot. Of course, few cases provide enough room inside the case to allow much more than one inch, so that's pretty much what everone uses.
If you can mount the fan *outside* the case, you get, of course get more distance. I have a rad with a pusher/puller setup, and the pusher fan is 1-1/2 inches away, and the puller is about two inches, but the puller fan is mounted outside the case.
Check out page 4 of the watercooling url in my sig.
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