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

feetmetersinches Hgmm Hg
-1000-30531788
-500-15230.5774
0029.9760
50015229.4746
100030528.9733
150045728.3720
200061027.8707
250076227.3694
300091426.8681
3500106726.3669
4000121925.8656
4500137225.4644
5000152424.9632
6000182924609
7000213423.1586
8000243822.2564
9000274321.4543
10000304820.6523
15000457216.9429
20000609613.8349
25000762011.1282
3000091448.89226
35000106687.04179
40000121925.52140
45000137164.28109
50000152403.2783