I found that you really need them to be touching the tops of your cheek to be effective. If anything can get up to your eye from under the lense, it's just like not wearing them at all. Protection must be total.
I learned this by cutting a piece of lumber with my saw and actually saw a piece of wood, just a small chip, come flying up under my saftey glasses, and into my eye!!!!!
I went to Home Depot and tried on all the saftey glasses that they had there and only found one pair that fit in such a way as to create a block of anything coming into my eye from under the lense.
In my case, it's always something small, light and fast that gets me. I've had all sorts of stuff taken out of my eyes, and it scares the heck out of me that one of these days I'm going to blind myself. I have three pairs of saftey glasses on my dashboard. I have twice as many on my workbench. I always wear them, and I still have accidents, but haven't in over two years when I went to the type that touches my face at the top of my cheek.
Eddie