Don't judge horn length and mass during mid-day heat waves. They always look heavier and longer in heat waves.
Don't try to hide in brush, ridge, or ravine after being spotted. Hiding after you've been spotted makes the antelope very nervous and they will run off. If you have been spotted but the antelope haven't run off yet, continue your stalk, angling toward but not directly at the antelope. Make a large sweeping arc, slowly closing the distance. They are very curious, and many times will allow you to walk to within rifle range as long as you are not heading directly at them.
Use a range finder. They are smaller than deer, and in wide open terrain range estimation can be difficult. A good range finder will take range estimation errors out of the equation.
If you blow a stalk on a good buck, don't give up on that buck or settle for a lesser buck. The good buck will return to his home range/territory. Patience is a virtue. This is especially true for hunting antelope.
Antelope are difficult to judge. Get some good videos and study them. The difference between a good buck and a great buck can be very subtle at 400 - 1000 yards through a spotting scope. Tony Grimmett's video, "Size Is Everything" is one of my favorites.
Hope this helps, Smokepoler63