I discovered a new use for my iPhone last night:
Start with the AR on a bipod with the butt supported on a block as close to horizontal as you can make it. This is just so the iPhone won’t slide off the rifle.
Level the rifle by placing the iPhone on the flat part of the scope rail. I set the bubble level app to a 10th of a degree readout for fine measurements. Make sure you get the side to side reading zero.
Once the rifle is level, move the iPhone up to a flat part of the scope and adjust the scope until the level zeros. Tighten the ring screws, alternating so the compression of the rings doesn’t move the scope. I left the iPhone on the scope while I did this so I could make sure I wasn’t moving it.