Putting our dog down. When is it time?

I recently went through this with 14 yr-old Ike, the most wonderful dog I've ever owned. I had always said "when the spark in his eyes is gone" or he "had no dignity" or "couldn't get around so that he could be near us" ( VERY important to an Aussie). But in the end, it was FEAR that made me take that final step. About 4 days prior, something "gave" in his left hip that made it hard for Ike to stand "squarely". He still hobbled around OK, but any steps were difficult and he fell far too often - even the single step down and back up from our deck, when he needed to potty. But he seemed rather content to hobble. The "spark" was still there and he COULD get around and his dignity was still somewhat intact. However, the nightmare that was haunting me, was Ike falling and really hurting something - something that would require the ending of his life under dire and painful circumstances, since we live 40 miles from emergency Veterinary help. His life was never going to improve; and I didn't want his last hour to be spent in pain and terror. So I guess I took the "coward's way out" - and although I still have a massive gray-and-white hole in my heart; I still think it was the right thing to do.