You can also go and get clip on sunglasses that fit your glasses, send them to Mike at Auralens to fit some Didydium or AUR92 lens in them and then you can clip them on your frames to work. So as long as you change prescriptions and not frames you will have no need to change your protective lenses. I do that with camera lenses and it works great. Mike is helpful like that.
Now, as far as minimum lense requirement for Frit, I am not a professional, but I think you would be fine with Didydiums or AUR92...Now, your eyes may be more sensitive so you may want to bump it up a notch, and I'm sure Mike would be glad to help you figure that out...
www.auralens.com seems like the best place to go and know you are getting an informed answer.
Good luck!!