.243 Win is fine for Pronghorn. And I applaud your bullet choice. Very few people understand that 85 grains is plenty good, and almost always more accurate than the heavier bullets. Don't let anyone push you into the 100-105 grain bullets.
I've only taken one Antelope with my .243; but it was more than enough for the animal, and the rifle goes on every hunt as a backup (I'm a .270 Win guy).
Remember: Antelope have massive hearts, livers, and lungs. Hit 'em in the butter zone, and they'll be dead before they stop running (if they even get started).
If Wyoming allowed .22 caliber centerfires, I wouldn't hesitate to use 50-53gr Barnes TSXs out of my .220 Swift for antelope. As Doodlebugger said, antelope are fragile creatures. It's all about shot placement, not energy (lb-ft) ratings.