Paul has shown the effectiveness of using a cutting/marking gauge prior to starting the plough groove as a way to prevent/minimize tear-out at the surface of the groove and keep it clean.
Regarding the initial question–in my (limited) experience, the set on the plough can often move slightly as it is being used and sometimes needs to be re-adjusted. However, I think you should set your depth stop and lock it in tight and not move it as you plane. The cutting edge should protrude slightly below the skates/runners/mouth as with any plane–you just may need to re-set the blade from time to time if it moves as you use it. And yes–start at the front edge of your work and gradually move backwards until you can run the plough the full length of the cut. Hopefully that helps.
This reply was modified 3 years, 12 months ago by David B.