All depends on your stock design and barrel contour. I want my scopes at what ever height my eye lines up the most from different shooting positions without having to move my head all over. In my little world of perfect I want the front housing to clear the barrel by two red curly ones stacked on top of each other. Any more than 5 I'm taking the router to the top of the stock where my cheek weld is. Easy way is to just stack some change until you get the alignment/clearance you want then measure the coins. Every company is different so a Md from Company A is a different height than Company B.