Photographing Rotated Reverse Coins.

I've always considered it a "standard" to orient the flipped coin exactly as you would normally, so that the reverse would present itself vertically if it still were. Used universally, that would allow anyone to tell the degree of rotation.

I know this is an old post but I had an Idea. If the coin is in a 2x2 you could draw a pencil line across the front center (on the cardboard only). Make sure to reach both edges of the flip. Then flip the 2x2 over and connect the ends of the lines from the front. I think this could give you a better measure of how far off the rotation is.

You could take it further and use a piece of paper with the flip and actually measure the degree of rotation.