I agree with Bradjoiner. In my mind the 3-tier is as much about minimal equipment as it is saving space. If you put the HLT on top not only can it be higher since you only need to be able to operate the drain valve, but you can take the hundred or so bucks you were going to spend on a pump and put it towards better equipment or more ingredients.
Base the stand height off of carboy(6.5G) height and valve clearance on the brew kettle. This puts the kettle bottom at ~28", another 2 feet to the top, 52"(4'4" ~belly button height) Another 2ft for the MT/10gal igloo(76", 6'4") and then a final 2 feet for the HLT puts you at 8'4. The MT is a bit inconvenient but only if you try to dump all the grain in at once. obviously when you clean it just tip it over.
If you don't mind or have to move the BK around for cooling anyways there's no reason you can't make it lower than carboy height.
1°: Cider; 2°: Blackberry Melomel, Banana wine; Conditioning:Orange/Clover blossom mead, Ginger Amber, Cyser, JAOM, In like a Lion RIS ; Drinking: British Mild, In like a Lion RIS