I would say 6" or so from the bottom would be fine. Mine is about that and it works fine. I checked it against a dairy floatey there and it was within a degree...
On Tap (Nitro): Black Sand Foreign Extra Stout
On Tap 1: Gold Dust Pale.On Tap 2: empty. . .On Tap 3: O'Rally Red
In Secondary: Moonless Midnight RIS
In Primary: Chocolate Palomino, Nightshade Dark Cascadian