Here is how I go about sizing up an antelope.
First, like was said above, so much of estimating dimensions is judgment. Judgment comes with experience, and is limited by the quality of bucks you have seen. Get some good glass and get out in the field during the cool hours of the day without heat waves and start making relative judgments between antelope.
Concentrate on mass, horn length and prong length. Rate each buck as follows; Mass - 1 to 5, Horn Length - 1 to 3, Prong Length - 1 or 2.
Notice the emphasis on mass. Be really stingy giving out 5. In 10 years I bet I have seen maybe 2 or 3 that fit that score out of many hundreds of bucks I have really looked closely at. A bigun will push 7 inch bases and have relatively heavy horn above the prong...beware heavy base and spindly top.
If you got a 10 then get to shooting, because you are looking at one of the biggest bucks in your area depending on how mucj time you spent afield. It has to be that simple because we just dont have the tools to measure an inch or two difference at several hundred yards.
If you dump a dink and are not happy, blame your lack of scouting. If you walk up to him and he starts to take your breath away then either you truly have done your homework, or you need to quit smoking.
Finally, mark a BLM or similar scale map with the highest scores you have seen during scouting in each 1 square mile section. Find that bad boy the night before season and tuck him in. Next morning at first shooting light be on him with the crosshairs, because Mr. Outfitter will be along shortly with his rich client to sling lead because you can bet Mr. O did his scouting.
Good Luck