I seek advice on how to choose the number of diopters for the flip-out magnifying lens. I know it will be positive, and not too strong. Maybe 1 or 2 diopters. I wear progressive bifocal glasses, and intend to keep them on for focusing. Using the flip-up lens to view the focusing screen, my glasses are almost effective enough, in the strongest parts, for good focusing. Also, I'd appreciate suggestions for sourcing a lens or two. Thanks. Larry.