... and puts the eye in propper alignment.
Watch out for that opinion. It's not a "proper" alignment unless the user has their rifle set up like your rifle.
If you're running a collapsible stock, short carbine or prefer your shooting form to have your face right next to the charge handle...then yes, you may need a cantilever optic mount.
This is not the case for all users so it depends how you prefer to have your optics on AR platforms.
Also, with a 50mm, getting it as low as possible is not always an option. The AR's are not designed like bolt actions, in that getting your objective as low as possible will greatly reduce you're ability to get your face low enough on the stock.
Your best bet would be to go into a gun store and have them mount something on an AR of similar design to yours, or simply bring yours in.
The PEPR is a fine mount, but just make sure you're getting it for the right reasons. It may or may not be needed.
I'm going to use Leupold Mark IV rings as a reference, so take that for what it's worth. I use a 56mm objective with EXTRA High rings and they barely clear a quad rail.
You should be fine with Medium if you're switching to a float tube.