Plumbing code requires that the low part of the spout be 1" higher then the flood level to prevent siphoning.
If I know there will be 4-1/4" tile, I put the spout at 4-1/4" from the top of the tub.
If the tile is 6", then I put it 6" above.
There isn't a real hard and fast rule to this.
Except for the 1" air gap.