Yes, you can. I just explained this method down in Mad Dogs Texas thread.
You have to know the maximum point blank range for your rifle. Most modern big game rifles such as the 30.06 will have a MPR of around 300 yards.
Thats a good figure to use anyway since when you shoot much further than that, wind play begins to have a much larger effect and an ethical hunter doesn't take chances with game animals.
Basically, you'll want to zero your rifle high at 100, close to dead on at 200 and it'll fall 3 inches or so low at 300 yards (this is about what you'll get with a 30.06 using 150 or 165 grain bullets).
You should experiment with your own rifle and determine its MPR yourself.
Now, a deers chest is 18 inches thick from top to bottom just behind the forelegs. Place an 18 inch object at 300 yard and the using your power setting on your scope, find the setting that perfectly brackets the object between the horizontal cross hair and the fat part of the vertical cross hair. Now look at the power setting. Wherever it is - say, its at "5" then you know from then on if when your variable scope is set at 5 and the deers chest is larger than the bracket you merely have to hold dead on and shoot. If the deer is only slightly smaller and you're in very windless conditions you might hold a tad high and shoot - you'd be very surpised at how much wind drift can effect a bullet and how much tougher it is to gauge among competition shooters than mere distance.
You'll have to find a similar common measurement for other game animals. I have no idea what a pronghorns chest measurement is. Maybe on hogs or some other game some sort of horizontal length measurement might be the ticket - I can only tell you about deer and caribou (caribou and elk are roughly 24 inches in the chest).
Anyway, whatever you hunt, take some measurement - don't forget that some animals may have a big disparity between males and females.
With deer, a doe is roughly 16 inches and really big buck is more like 20 inches - If a deer is right at the maximum range of my bracket I might hold a a couple inches high or low depending on whether its a doe or a big buck.
Play with your rifle at long range and work out your own MPR formula's for your scope - each scope and load will be different, theres no common settings.
Have fun, impress your hunting partners!
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Keith
The Bears and Bear Maulings Page:
members.xoom.com/keithrogan