I've been following a natural vision improvement program and have finally decided that I need to give in and order a reduced prescription pair of lenses. The problem is that I don't want to spend very much money on getting a new pair of temporary glasses. Also, I've heard it's a pain to get conventional eye doctors to agree to stepping down your prescription.

I've found sites online where you can order cheap glasses if you know your prescription.

If I know my current prescription, how do I alter the numbers so I can get a reduced pair from one of these online shops? I figure altering the diopters isn't too hard, but do I change anything for the astigmatism?