I got fed up with playing with levels, and purchased a Segway reticle leveler.
Boy, am I glad that I did! I still have all the levels and so on, taking up space in a drawer. I seriously doubt that I'll ever go through the trouble to use them again. The Segway is a lot faster - and does a better job. The Segway reticle leveler at Amazon.com The Segway tactical reticle leveler at Amazon.com
The Segway has a level on it, but I generally ignore it. It is really not necessary.
Yes, I know... It looks like a cheap POS. - But try one out a few times and you'll see why I do not bother with levels any more.
If you read the reviews at Amazon, you'll note that users either love of hate it - depending upon how good they are at reading and following the simple directions.
There are a couple of videos on YouTube that should enable even the most knowledgeable 'firearms expert' to work out how to use the thing. The video put out by Segway lasts ten seconds... - That's how simple and accurate the device really is.
Fellows who have invested big bucks on Rube Goldberg contraptions with multiple levels may disparage the Segway, but I think that we can safely ignore the "sour grapes" crowd who do not want to admit to themselves or others how thoroughly they've been gulled.
A word of advice here: Put the rifle up at eye level, whether you are standing or sitting, so that you are comfortable while using the leveler. My son tried to use it with the rifle in the work cradle, and he looked like he was practicing to be a contortionist. This is probably the single most common mistake in utilizing the Segway. If it is not easy and comfortable to use, then you are doing something wrong.