New
#21
I have my top fan on my new build blowing into the case. This is in front of my CPU cooler which blows out to the rear exhaust fan. The only other fan I have right now is the front, which cools the hard drives, so the air is warm from it is warmed before reaching the CPU. I figured I needed a source of fresh cool air for input into the CPU cooler fan.
I may try it as input from the bottom of the case (less noise as I can seal the top of the case with a soundproof plug.
So I think it all depends. There isn't a formula - it depends on your case geometry and the other fan placements.