Bit confused about dioptres for my Hexar and hope someone can help.

As I understand it the Hexar is a base -1 dioptre which allowing for my eyesight at +2 means I'll need a + 3 dioptre. I assume I'm correct here?

Japan Exposures have a magnifier attachment available which seemingly accommodates a Leica dioptre BUT two are available. 1.25 and 1.35. Which one should I buy? And does the Leica dioptre actually work in the unit?

japan exposures | films and more (formerly Megaperls Japan Webshop)

Anyone using such units successfully?

I shoot a 35, 50 and 90mm lens if this is relevant.

Thanks for any input