For reference, a 143 ELD-X with a MV of 2,720 fps and a sight height of 2-inches needs to have a cant differential of 10 degrees to see a horizontal differential error of 0.75" at 100 yards......that's a boat load.
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Below is a picture showing a vertical line and then a 5-degree cant on either side of the vertical line. You'd have to bounce between either -5 degree to the +5 degree cant to add 0.75" to the horizontal spread of the group.
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You should be able to tell that you are within about 1-degree without a level or device. The horizontal differential from a +1 to -1 degree cant for the above example at 100 yards would be 0.154" (i.e. 0.147 MOA). To gain better resolution, you'd really need a Send-it level (or similar).