Many ways for solving.
1. Decide the amount of cables that will be running there and the needed height(When I get the value, I'll multiply it with 1.2-1.3, just for future reservation)
2. If you have preliminary info about the cables, just select raised floor system(if you use a ready one, but not building it on your own) and check which type of "legs" for the floor will match your needs
3. Even though the answer "7 feet" is the best, you can always check, or change a cable without bending or crawling.
If a large project, just calculate some minimal height and let the architect chose the rest