Alright guys, here's a little test:
Most of you will be looking at this through a smart phone. Hold your phone at arm's length and look at the image below. Can you recognize which line is plumb and which line is not? Now rotate your phone counter clockwise until the canted line looks plumb and look at the top of your phone. Can you tell that the top of your phone is not horizontal?
I'm not saying never use a level. But a simple look at the top of your turret is getting you very close. Not to mention the levels that are built into our ears/eyes/brain. You shouldn't need a level at 300 or 600 yards regardless of environment. I've shot to 1,400 yards without a level and was just fine.
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Here's another tip: when shooting paper, first make a mark on the target backer with a 4-ft construction level (after you check the level to make sure it is correct). This way, everytime you shoot paper you are forcing yourself to be behind a rifle that is not canted. Pretty soon you will be able to know what right feels right, and you won't need a device to tell you what you should already know.