Well I made a post and it disappeared. Maybe it will show up later.
Anyway I have never used a precision M-2. Altimeters measure the change in barometric pressure which causes the needle to move which shows the change in altitude (elevation) on the dial. To get accurate readings you need to find a known elevation and adjust the dial to read that elevation under the needle. That little +/- slider is probably what you use. As long as the weather does not change you will have 4-5 hours to get accurate readings then you should check another known elevation.
I have used smaller less accurate altimeters when hiking and using a map that shows the trail and elevation contours. At the parking lot I would find an elevation and adjust the altimeter. Then I could find where I was on the trail by reading the elevation on the altimeter and finding the closest elevation contour on the map nearest the trail.
here's a couple links that might help