Check out this web page for some possibilities. My 338 has a 28 inch barrel and I guess I haven't chronied it yet. I know that 5" high at 100 gives me a zero at 440 from the time I checked at the range. I just use it out to 500 yards on elk and know about where to hold for those ranges.
Some advice that you can consider as you wish though... I don't know exactly what you mean by a "guest gun". If you mean allowing others that are unfamiliar with that gun to use it, you might consider a muzzle brake. You wouldn't want to have someone dot their eye with the scope of this beast.