Hope someone can help. I am trying to order a pair of glasses on line and I have my prescription. I need to know if I can get away with a 1.57 index as opposed to 1.61 or 1.67 index. My bad eye is -4.75. How thick would they really be? I am cheaping out because they are only warm during allergy season or when I take the contact lenses out at night.

Thank you!