Thanks for the well thought out and expressed view.I have used them mounted on the scope, rail, and tang in the case of my MPA chassis which come with a built in bubble level. I very much prefer the level mounted off the left side of the scope. Two reasons. First, there is minimal head/eye shift between the scope/ target when shooting. Second. Being attached and properly aligned with the scope, I can leave it in the position permanently knowing the crosshairs will be perpendicular when the bubble is centered.
That's a sweet setup. Gotta have the rail space available, though. I generally am lacking real estate for something like that at the back of my gun. For all of the options that mount or reside under the ocular lens, that's probably the coolest one that I have seen.Generally the level should be mounted on the opposite side from your dominant eye, far enough forward to see clearly, so you can use you off eye to check level without loosing your sight picture. This works really well.
However, mounting off to either side does tend to get in the way, so levels that flip out may be the answer here.
My left eye has a retinal problem so I must use my right eye to check level as well as sight.
I found a rail mounted level from Knights Armament, that fits under my scope and I can momentarily glance down with my right eye to check it. I really like it and have gone to this on my rifles that have enough pic rail to do this. View attachment 243159