I thought I'd add some controversy to this post! I have been hunting antelope since I was old enough to hold a rifle and still seem to get fooled from time to time judging them! One thing I've learned through the years for both deer and antelope is not to use body parts to come up with measurements. Each animal usually has a little different body comfirmation and this can change depending upon what angle you are viewing them and which way their head is turned (upright, side view, etc)! Obviously it is very important to get different views and angles of a buck before shooting him!
As an example, if you look at the antelope photos posted above just about every one of the bucks in the photo's ears are cocked somewhat backwards. You may not be able to pick this up from the front view but will likely see this from the side. If you are below or above a buck it will also make the ears look longer or shorter. I haven't really been a fan of using ears, eyes, length of nose for field judging...more times than not this has lead to ground shrinkage!
I put a tape on a bunch of my antelope mounts. Just about every one of my bucks has 4 1/2" ears from the tip to the where the ear attached to the inside crown of the skull. The outside of the ear where it connected (about even with the eye) was close to 6". My tape stretched to 7" if I add on the additional ear parts that are attached along the neck. The 7" measurement would be nearly impossible to see from a distance or any view unless you were at point-blank range. I'm not sure if NMex antelope have longer ears than Wyo antelope but that may be something to consider if you are set on using ears to estimate scores.
One thing I have always used for scoring antelope is visualizing a dollar bill. A dollar is exactly 6" in length. If you look at the giant pronged buck in the first photo you can definitely tell his prongs are well over a dollar bill in length!
One thing you may also want to remember is that overall horn length is only a fraction of the score compared to mass measurements! A lot of guys seem to get caught up in horn length! Obviously it takes pretty much everything to make a B&C buck! Different parts of the country usually have particular horn confirmations for bucks that qualify for B&C. If you are familiar with these particular measurements it will definitely help make your selection of bucks a little easier (if you are searching for B&C!). This same thing often holds true for deer, elk, moose, mtn goat, bighorns, etc!