Anyone have a quick 'n dirty way of guessing which diopter strength to use? For example, do camera diopter numbers have some rough relationship to those dime-store reading glasses with strengths like 1.5, 1.75, 2, etc.? Or can something be guessed by where I would set the diopter setting on an SLR with an adjustment slider/dial? For example, with my normal glasses, I have to move the slider on my D70's viewfinder almost all the way to the top. On the other hand, with the glasses I had made for computer work the slider is almost at the bottom. Would ZERO on the diopter scale be like putting the slider in the middle? If so, would UP be POSITIVE or NEGATIVE?