Basically, the higher altitude you are the less vacuum you can attain. Always work with the maximum vacuum and when in doubt, pour a small test mold.

Altitude Above Sea Level

Maximum Vacuum

feet meters inches Hg mm Hg
-1000 -305 31 788
-500 -152 30.5 774
0 0 29.9 760
500 152 29.4 746
1000 305 28.9 733
1500 457 28.3 720
2000 610 27.8 707
2500 762 27.3 694
3000 914 26.8 681
3500 1067 26.3 669
4000 1219 25.8 656
4500 1372 25.4 644
5000 1524 24.9 632
6000 1829 24 609
7000 2134 23.1 586
8000 2438 22.2 564
9000 2743 21.4 543
10000 3048 20.6 523
15000 4572 16.9 429
20000 6096 13.8 349
25000 7620 11.1 282
30000 9144 8.89 226
35000 10668 7.04 179
40000 12192 5.52 140
45000 13716 4.28 109
50000 15240 3.27 83