I've dealt with the same thing for years. I do all of my range work at 5-6000' elevation, but then go hunt marmots at an average elevation of 12000'. It was no problem holding minute of elk out to 700 or so, but I really got tired of missing by mere inches trying to shoot minute of marmot. This year I started using a G7 rangefinder
, and after getting the ballistic inputs fine tuned it is giving me spot on corrections out past 1000 yards.
You might try this; get the usual ballistic data, but also record your atmospheric data at the same time. Go to G7's ballistic calculator and enter it all in. When it matches your results in the field, change your elevation in the calculator and it will automatically change your air pressure , and temperature. Correct the temperature, and the results should be pretty close for the higher altitude.
In the mean time save your change for the G7 rangefinder.