It really all depends on how you make up your dip tube on the inside. If you are going with a false bottom, then you need to base it off of the height of the false bottom and dip tube. If you are using a pick up tube with something like a HopStopper or bazooka tube then you need to measure it out from there.
At the very least i would go a little higher if you want room to change things around, you can always make a longer dip tube, just make sure that you have a good seal on all your joints to keep a good siphon going.
"Yes, I am a pirate two hundred years too late. The cannons don't thunder there's nothin' to plunder,
I'm an [under] forty victim of fate, Arriving too late, arriving too late."