About 1 inch is around the right spot, but the most important thing is too make sure your entire coupling is on a level spot. Meaning you must get it above the bottom curvature near that bottom weld.
Most false bottoms sit pretty low in the keg, so you should not have a problem with the diptube there. But I would certainly wait until you get your false bottom to test fit it. It's really hard to patch a hole in a keg!
And as long as you have a dip tube, and your solder joints are good, it really does not matter how high off the bottom you valve is (within reason).