Level is a very loose term, since everyone holds a firearm differently. I set mine in a lead sled and get it close. Then I shoulder it with my eyes closed. When I open my eyes, I should have a full field of view and the cross hairs squared to my view.
I see guys putting a torpedo level against the frame of the gun or laser sighting the top dead center based on the scope mount bore holes. For me, that doesn’t work. I hold all my rifles slightly counter clockwise. My daughter is just the opposite and holds slightlty clockwise to top dead center. Just my opinion, but the true top dead center is the TDC of the barrel is where it is at at the time I squeeze the trigger.