If he verified with kestrel then it already uses the correct atmospherics. No need to confuse anyone about station pressure or density of air.
More about that in a second.
BC won't have a extreme effect from the off the box. Not at 600. Maybe at 1k but that around 1 moa most between .300 and .311 at 1k.
The problem more than likely lies in the weird system the OP is using with 2 inch high @ 100 but zero'd at 255 and the the data input into the system. Another problem could be using hornady app? If that's the case, and I could be wrong, but I'm pretty sure they only use hornady bullets…which inherently use hornady bullet form factors.
However, to revisit the atmospherics, I don't think you can have a station pressure of 26.45 in that temp at that altitude unless you were shooting in the eye of a major storm…
My logic based on a simple DA chart as admittedly I'm not a meteorologist.
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