This is how I do it.
Turn the rear sight down until it bottoms out.
Turn it back up 10 clicks- this should get you pretty close, though it varies from one rifle to another. Shoot and adjust until you are zeroed- move the rear sight in the direction the bullets need to go- move it up to move your shots up, and right to move your shots right etc.
Now turn your rear sight all the way back down and count the clicks. Wrie down the number of clicks, this will serve as a reference in the future. If you picked your rifle up and wanted to check its zero, you would turn the sight all the way down and count the clicks back up. If it was at 13 clicks from the bottom when you zeroed it, you would count 14 clicks up, then click it back down once to make sure the sight moved all of the way.
The Elevation drum can be set also, by loosening the screw and aligning the mark to your zeroed distance. I can't rememeber if M1s have a mark for 100 yards, but it will be 2 clicks down from the 200 yard setting.