Depends on your sand. If it's PFS or a medium to heavy grain size about 2" above the sand is fine. Keep in mind the default "strainer" over the intake will put you another 4" or so above the sand just from it's height. If you not using the default "strainer" or have really fine sand I would in the area 6"-8" above the sand. I have both and operate mine a little lower. It's just my preference but the above is a safe bet.
When do we get pics of your project and what your trying to do? Now you got me curious..
@Jgray152 what shakin bro, have not seen you around in a while. Glad to see you back