INAppy, I am sorry for your loss. I have been following this thread because my mare (23 yrs old) also has ERU and has lost her sight. Unlike your beloved Tell, T still goes into the pasture and appears "normal".....that is until she walks into a tree or trots into the corner post of the paddock. So far she has not seriously injured herself. I , like you, am struggling with the decision. I have ridden her in the field and part way down the trail, and while she handled it well, I am in constant alert mode. I wish I knew if she was in pain, for that would aid me in making a decision. Right now I am on the fence about what to do. I watch for signs that she is ready to cross, but I see none.
I admire you for doing what was best for your dear boy. I know it was not an easy decision, as I am currently struggling with it now.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".