Some of the stuff above is very good. My personal thoughts on a rally ball:
1. Shot should be hit at about 75% of full effort (100%).
2. Ball should have enough topspin that it can clear the net by 3-5 feet and land 3 feet or more inside the baseline
3. Player should be able to make it 90%+ of the times it is attempted
For juniors 12 and under the elevation may be even higher than 3-5 feet. My attributes above are for adults.