When i emailed their customer services a few months ago; they said 6-10". I'd probably go with an 8-10" height; so your sump should be perfect.
Ideally you'd want the sump in the first compartment (in the compartment where the DT overflow is coming into). Your fuge will come after your skimmer. This comes in handy if you plan on growing pods
in your fuge. Since your skimmer is before your fuge; they won't die off in the skimmer etc.
If you decide to put your skimmer close to the compartment where the return pump is located; just be careful of micro bubbles which may get pumped into the DT. A bubble trap will help if needed.