If the Leica system is -.5 then you'd want a -2 diopter to get the -1.5 seems to me.
No, I think the numbers on minus diopters are simply combined to give a total figure. Plus diopter close up lenses work that way, i.e. If you add a +2 and a +3 close up lens you have a combined diopter of about +5, it isn't exactly +5 because there is some spacing between the lens elements, but it is close.
So then, let's say the M3 native correction is -.5 (I'm just guessing, I actually don't know and I'm not sure how a person would find out for sure.) But if it was -.5 and then you obtained a Leica M eyepiece correction marked -1.5 and handed it to an optometrist to check on his tester he would find it to be a -1, which would combine with your cameras viewfinder to give you a total correction of -1.5 Does anyone here know if Leica published that information about their viewfinders? But I really don't think it matters, if you put a -1.5 on your M3, then that will be your total correction. It wouldn't make common sense for Leica to label them any other way.
Finally, this will be a 'cut and try' sort of thing, I don't think you will know for sure until you actually try it out. There is however one more option. If you know a friendly eye doctor, tell him what you are trying to accomplish and why. Perhaps he'll let you try test diopters, holding them to the eyepiece, to see what works. In fact, are there any optometrist at APUG who are also Leica M users who can offer advice?
