The eyepiece viewing system of an SLR or rangefinder typically is designed to produce an image whose apparent distance is about 1 meter from your eye - closer than the actual object, but much larger than the physical distance to the screen or RF spot.
It's different than trying to look directly at the screen of a TLR, where you really are trying to look directly at an image on the groundglass a few inches from your eye. I can still use an SLR without glasses, but I can't focus a TLR without using either glasses or the pop-up magnifier.
Camera manufacturers often label the eyepiece screw-on diopters with the value that corresponds to your prescription, not the actual diopter of the eyepiece. So if you use +2 reading glasses, you buy the +2 Nikon diopter. It alone is not a +2, but it combines with the existing eyepiece to give you the proper correction. I don't know if Leica uses a similar numbering system, but a Leica dealer probably knows.
I know this is heresy, but IIRC some of the former-Soviet screwmount rangefinders have an adjustable eyepiece diopter built in.