I use a "5", which is 2.5X @ 8". (Nice rundown of the available magnifications on Donegan's product page:
http://www.doneganoptical.com/products/optivisor.)
I personally am reluctant to go much stronger than that. I've had issues with the limited field of view when reaching for tools, etc., including near-misses grabbing the wrong end of a soldering iron and/or Xacto knives. Also, going much stronger than "5" means the work has to be really, really close to your nose, and at that point the visor itself can start knocking things over or breaking stuff if you're, say, trying to work within completed scenes. That said I may have to get a "10" (or spring for the add-on loupe) for dealing with the 0402 LEDs I've been playing with recently.
FWIW, the cheap knock-offs you'll find at the tool vendors at shows aren't worth messing with. I've tried two and they were wastes of perfectly good train money - optics were horrid. You will not regret buying the real thing.
YMMV, of course. Temper my experience with that I currently need 1.75X "cheaters" for reading or general close work.
