Vacuum at different altitude
Because you are manipulating the pressure using a vacuum pump, the altitude change should not matter. If you look at the phase diagram for water in the attached link, you can see that if you keep your system below 0C, say at -10C, than you will cross the solid-vapor equilibrium line and your ice will start to vaporize when the vacuum pump decreases the pressure to ~200 pa.

https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phase_diagram_of_water.svg

To convert from pa to in Hg, I suggest searching for the conversion factor. From google: 1 in Hg = 3386.39 pa. So if you have a reading of say, 2 in Hg on your gauge then: $ 2 inHg*(3386.9 pa)/(1 in Hg) = 6772.78 pa $