Sevo's point is that the power marked on the eyepiece may not refer to the eyepiece itself, but to the eyepiece + prism; thus, the final, or effective, power with the eyepiece in place. For example, a Nikon finder has a native power of -1 diopter, even with no eyepiece in place. So the standard eyepiece, which is window glass, gives an effective power of minus 1. The reason is that the Nikon finder presents the image at a virtual distance of one meter. A minus one diopter power is consistent with the image being presented at that distance, for most of us.
Now to apply this to your (non-Nikon) camera: if the standard eyepiece is -1.5 diopter, that means the virtual image in the finder is at a distance of 66.67 centimeters. So the minus 1.5 diopter eyepiece will allow a normal eye (or one corrected with eyeglasses) to focus correctly in the finder.
Next, what eyeglass prescription do you need for normal distance vision? Do you require any correction, or do you have normal distance vision? Or do you wear glasses to see at a distance? That's the place to start. I think you may be overthinking this by bringing your reading glasses into it. You shouldn't be using them with your camera to begin with. You should either look into the finder through your distance prescription (if you have one); or simply with your normal, uncorrected eye.
I hope this helps. Let us know.
