Locate the balance point of the rifle by holding it at your side, shift it back and forth in your hand until it balances nicely. The sight will not go there, it will be either forward or to the rear of that point, you don't want it digging into the palm of your hand when being carried. Very likely it will be forward of the balance point given "old eyes." Mark the forward limit of the balance point, then tape or somehow temporarily attach the sight beyond that so you can look down the barrel, adjusting the position of the sight, and find the spot where it is "most" in focus, the place where you get a sight picture you can live with. That will be your sweet spot for that sight. Install.
dave