viking,

You're obviously on the Internet. Here's a good formula for figuring it out:
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

The only problem is the "pressure" box. This refers to atmospheric pressure, which varies primarily with altitude. That's pretty simple: Just use 1 digit less for each 1000 feet above sea level. The standard is 29.92, so use 27 for 3000 feet above sea level, or 25 for 5000, etc.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck