I remember reading this a long time ago, and as usual Randy Ulmer is thorough and easily understood. I found this on Archery Talk:
Here is Randy Ulmers take on the subject as well as a quick fix while in camp.
At high elevation the air is thinner and gravity loses a small percentage of its pull. Every projectile slows more gradually and takes a flatter line. While baseball’s power hitters love the thin air of Denver’s Coors Field and professional golfers enjoy 10% more distance at Castle Pines just south of the city, this phenomenon is not a bowhunter’s friend. Sure, elevation flattens trajectory, reducing the effects of misjudged shot range, but all too often the knowledge comes as a surprise to the hunter at just the wrong time. If you hunt more than two thousand feet above the elevation where your bow was sighted in you will begin to notice that your arrows impact higher than normal. This can be a serious problem for bowhunters heading for the timberline from eastern states.
The difference can be fairly large, several inches for arrows fletched with vanes and even more for arrows decked with feathers. Left unchecked, such a built-in accuracy bias can produce an outright miss, or worse.
Of course the real key to remedying the problem is first understanding that it exists. After that, the physical solution is simple. Because the gap between the pins is affected, you can’t simply gang-adjust your entire sight head upward to account for the flatter trajectory. And, resetting every pin individually is too problematic in the constraints of most hunting camps. Instead, focus on one of your longest pin settings. Assuming it is 40 or 50 yards, set out a target at that distance and shoot until you are comfortable that you are executing good shots. Then simply turn your limb bolts out (reducing draw weight) in small increments until you are hitting dead-on at this range. Make sure to turn both bolts an equal amount in order to preserve the bow’s tiller setting.
After making these adjustments move up and check your shorter-range pins just to be sure everything is still tracking. Your 20-yard impact point may be slightly below the intended target, but the difference will be so small that it is insignificant.
Not only is elevation an important consideration when preparing yourself physically for the mountain hunt of a lifetime, it must also be considered when preparing your bow. Adjusting your draw weight is a
step that can make a big difference in the outcome of the hunt.
Caption: Hunting at altitude will affect your arrow’s point of impact. A small adjustment to your draw weight can quickly fix the problem.
but I do have a few documents that say I'm an engineer. However, if you don't use it you lose it, and I never do these calcs anymore and rarely think about them. This was stuff I survived back in the 90s. I don't think I'm off on my numbers based on velocity, but some of my reasoning may be off. A good student in college would eat my lunch on these calcs.
