Hi, Jeff. Not to be smart but leave a reveal above the opening the same as or more than on the sides, add in for the horizontal millwork that your cabinet maker is going to do. That is your mantle height. There really is no standard, as there is not a real standard opening. The fireplace stores have equipment for many sizes of screens and glass, and as soon as someone tries to set a standard then everybody wants a Rumford fireplace which is totally different. I do hav e a suggestion, though. Go to your home center and look at their surrounds. Check their dimensions and go from there.