You are correct that it would be an 18 yard shot from the base of the tree and that is what you should hold. The height of the shooting position is a factor in shot placement though: theoretically you would want to hold high to angle down through the center of the vital area. I would recommend this for elk and bear, but on deer, I would hold center of the chest or even slightly low because the are notorious for jumping the string and the initial moment is down.

I would practice- at least shoot a pine cone with a blunt or field point once from the stand. One arrow will answer many questions that otherwise are discovered in the heat of the moment. (such as limb clearance)


Good luck!
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In the legend of King Arthur, the Fisher King was a renowned angler whose errant ways caused him to be struck dumb in the presence of the sacred chalice. I am no great fisherman, and a steelhead is not the covenant of Christ, but with each of these fish I am rendered speechless.