I would make your thighs as tight as needed. And by that i mean loose thighs look a bit crap but do them too tight and you wont be able to walk, run or sit down or go up stairs.
With the fx i would try to tapper them in at the knee so they look less like bermuda shorts and dont knock at your knees. I have mine fairly tight i could maybe get a finger inside but how high? Remember they will rise when you walk, mine at a standstill look fairly low but they touch my ab plate when im in full stride.
I would measure, tape them up and try, then adjust again til your happy lots of times before glueing. If are intending on using velcro for all closure you will have a lot more room to play.
Anyway i hope this was of some help for you if you need to know anything else just shoot