You are correct that altitude affects idle vacuum, but I don't think you are in the middle of the good range.
Several of my trucks live at 6250 ft elevation and they are all tuned between 16 and 17 inches at idle (as opposed to normal 20-21 at sea level). Based on that, I would think you should be closer to 15-16.
At 1000 ft elevation you may lose close to 1 inch of vacuum (compared to sea level), but as you go up in elevation each 1000 feet of change will be less and less. At 8000 ft you are above about 1/4 of the earth's atmosphere and the difference between 10,000 ft and 11,000 ft will be relatively little. My engineer friends tell me that for each 1000 ft you lose 3% of power due to less air pressure=less oxygen for combustion.
Kent
