I had a very similar conversation with a woman at work. She had a pair of script glasses that had not been made to take into consideration the size / curve of the lens of her glasses.
I never thought about it, however my optician recommended a pair of glasses for me and when orderring the len inserts used all his knowledge to provide me with a pair that was right for me. However he doesn't come cheap, but I spend money on things that I want to be right first time. In the end I find that it pays for itself.
I remember when I was a child and my mother tripping up because her glasses / sunglasses prescription was wrong. I guess it's a science afterall.
Hope those new shades work well for you. It really does make a difference when things are done right.
Some things are cheap because they work and have worked for years. Some things are expensive because you are paying for the name. Unfortunately I am paying for the name on my prescription running shades. That however, does mean that they work.