Under the UPC, the mouth of the spout must be at least 1" above the flood rim of the tub. That is a little low from an aesthetic standpoint, and 2 to 4 inches is more the norm, from that standpoint.
The minimum distance down from the valve is to minimize the backpressure at the valve, which if too high, causes some water to dribble from the shower head when using the spout. Same reason the pipe to the spout needs to be full size 1/2 copper or 1/2 brass, but NOT 1/2 CPVC or PEX.





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