This is only if you are using google earth etc. to judge range. If you are using a LRF then the distance it spits out is precisely the distance between the LRF and where the IR splash hits regardless of angle.
The change when shooting at major angles up or down is that the ballistic curve acts very differently due to the change of the angle the gravitational pull has on the projectile. Over very shot ranges like the OP is posting about this should be pretty minimal and might only shift the POI by an inch, not causing total misses.
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