Choosing an eyesight correction dioptre can be tricky even if you know what's optimum for you with another camera. Reasons are
1: different cameras have some subtle differences in their viewfinders' "built in" dioptre, and
2: there is a difference between an SLR and a rangefinder. With an SLR you're always focusing on a screen at an invariable distance from your eye, but in a rangefinder you're actually looking at the subject itself through an optical system and there's a difference between near and far subject distances.
If you can, try a range of values in a store, preferably in dim light where your eyes have minimal depth of field making the choice more critical.
Also, if you're young, your eyes have a greater range of focussing ability. I am a bit short sighted and although my optical prescription for distance vision has hardly changed since i was a teenager, the ability of my eyes to cope with a slightly suboptimal diopter has reduced to about zero. Even between ages 55 to 60 it got worse. So at least in my case it can't get any worse than that.
If you can't get several to try, my best guess would be to go for a bit less on the Leica. They have values at 0.5 dioptre increments, so -1.5 could be it. I think that a bit less correction is easier to deal with than too much.
Another thing: my understanding is that all viewfinders have -1.0 approx built in. For most makes, the diopter value is added to/ subtracted from the existing built in value. For Nikon SLRs, they call the diopter the total value (at least that's my understanding). FWIW, my Nikon film SLR is best when -2.5 is dialled in, Leica M6 best value is -1.5, Mamiya M6 best value is -1.0 (half units not available) and my eyeglass prescription is -1.5 (and -0.25 cylindrical which has to be ignored).
Hope this helps.
