Ray,
Just a little food for thought based on my experiences last year when I slipped a disc and mobility was compromised to say the least.
The angle finder worked fine for me when shooting horizontally or inclined upwards. It actually became a disadvantage when shooting inclined downwards as you have to lean over further to see what you're doing.
In short, if it's your back that's the issue then it won't help in all situations but if your knees/legs are letting you down, and bending from the waist isn't a big issue, then it would work well.
I'm not sure what your budget is but I've got a "video based" angle finder that is a small camera that fits on the viewfinder and has a remote screen to monitor what you're shooting....brilliant for all situations but comes in around £200.
Bob