....I am wondering what a safe distance from the water surface or sandbed is to prevent sucking air or sand at your suggested flow rates for the 3/4 nozzle.
I have never tested distance for the sand bed, but I would assume it similar tp the the water surface - so if you keep it 3/4in to 1in from sand bead (or three surface') then you should be good.
Here's quickstart guide to your you started in terms of how best to position