your prescription - (100 / desired distance in cm). 

As a matter of fact I'm using glasses I bought this way right now. I actually find these even more comfortable than my full degree glasses for all-day use too, in terms of eye strain.

I feel it's ridiculous that the medical industry thinks I need to wear glasses that are powerful enough to focus an image of a mountain 100 kms away all day, when my average focus distance throughout the day is probably less than one meter.

These reduced degree glasses are called differentials. If you want to go into the rabbit hole of fixing your myopia, start here: https://www.youtube.com/watch?v=XPIGDSY_xBs

Disclaimer: When driving, you have to obviously wear your full prescription glasses.