I can no longer read easily at normal reading distance so checking focus by the naked eye is really out of the question but with a Paterson grain focuser it makes no difference whether I use glasses or not except to the extent that the focus thread across the top of the Paterson eyepiece would have to be altered depending on whether I use glasses or not.
The point I am making here is that once the Paterson is set up for either glasses or naked eye the grain will come into focus either way albeit the sliding focus section will need to be set differently for glasses and without glasses.
I had thought that all grain focusers work the same way and thus getting the grain into focus is independent on your visual acuity once the thread is correctly sharp.
I can of course get the neg into perfect focus with the grain focuser without glasses but should I then take my eye away from the eyepiece and look at the neg projection it will appear fuzzy but it is my "old eyes" that are at fault and not the grain focuser.
If I were to move my eye further away from the eyepiece then eventually the neg becomes clearer in the same way that a newspaper become clearer if I hold the paper at arms length and not the usual 14 inches normal reading distance
I suspect your problem stems from this as well. However if you use the focus thread or whatever the Magnasight uses for your eye and focus this without glasses then the grain will come into focus at that point that the neg is perfectly focused.
So it should be possible to focus the Magnasight with your eye close to the eyepiece and get the neg in perfect focus.
When the grain is in focus and you take your eye away then ignore the fact the the projected neg will then appear to be slightly fuzzy due to your eyesight. It isn't fuzzy of course it is your eyes that aren't working as they once did when you were in your youth