i'm one that will totally disagree. i find more holes in the canopy the higher i go. it keeps my scent above the deer when they are in bow range. less chance of them picking me out. and i can see farther. but to each their own. go with what you are comfortable with.Have you checked with Gander Mountain or Bass Pro shops for safety harness?
If you're able to sit still and scent down really well I say 12' - 18' is perfect a lot of others may say to go a lot higher but when you begin to go above the 18' range you're entering into the natural umbrella of the woods and limiting your viewable range. I believe it's better to see your hunting area as best you can but If you go too high with your treestand you're going to miss some action. Good Luck