Typically, most have their units around 7-10inches. With PURE T5's I've seen the range is typically 5-8inches. However, with the LED's as well, you are going to have CRAZY PAR, so you can easily hang the unit 7-10inches to get the coverage you require.
I have an 8tube 24" unit 8" above the waterline on a 30" long 47G tank. My unit is overkill for my tank, but I bought it with an upgrade in mind for next year.
EDIT: Whichever unit you get, remember you have complete control over how bright you run the LED's, so its not so much as how high, but more about what height will get you best coverage front to back and sides etc.
I get around 350-400 PAR on the bottom of my tank! Thats with Royal Blues running 100% and the others less than 30%.