Having spent a significant portion of the last ten years of my life writing about, photographing, and measuring big mule deer, I feel like I have a pretty good grasp on this one.
How to measure height: from the skull straight up(staying in the center of the rack) to a point even with the highest tine (i.e. put the rack up against a square wall perfectly perpendicular to the axis of the skull)and measuring from the skull to the wall, staying central to the center of the rack.
Buckspy is correct in saying that 27 inches is almost mythical. 27" tall is the equivalent of saying 45 inches wide. People throw out numbers (how many 30" tall bucks have you heard of) but rarely know what goes into those numbers. The longest tine EVER MEASURED ON ANY MULE DEER IN HISTORY is 24-6/8, and only a small handful 23" and up. Add to this a small amount of height from the base to the G-2 and you can see why 25" is really phenomenal.
The tallest rack I've ever seen in person is the Cary Cada buck (p.140 in Idaho's Greatest Mule Deer) and it is only 26" tall, and that includes a 24-1/8" G-2!!!
To reach 26-28", not only must a rack have a G-2 in the low to mid-20s, but also an oddly shaped lower rack that angles up quickly. Not saying a 30-inch tall buck isn't possible, but I have yet to see one.