I have never had an adjustable but just bought a slim and a fat boy last night. For those of you who have one, do you tend to find your number and just leave it there, or do you actually use different settings ?
I get irritation on my neck occasionally so I tried doing a 3 pass shave and adjusting between each pass. WTG on 7, XTG on 5 and ATG on 3. It actually worked really well but I don't do it that often.
I leave both of mine on seven for all three passes. I messed around with the different settings at the beginning, but found what I like so I stick with it.
I did the 7-5-3 thing with my Slim a few times. It worked well. Then once I forgot that it was set to 3 and just shaved all my passes at that setting. I still had a nice shave. I think most times I go with either a 5 or a 7.
So many choices! I'd advise starting with a setting similar to a razor you're already comfortable with. Gillette Techs are probably similar to 2 or 3 on a Slim or FB. EJ and Muhle 89's are more like 3 or 4, while Merkur HD seems similar to 4 or 5. These are my subjective, personal experiences, so YMMV. More quantitative guidance can be found on the Blade Gap Wiki.
After several shaves at a single setting, you'll be ready to start experimenting. There are many B&Bers who start high and dial it down for later passes, and some of us who do the opposite. I wouldn't suggest making big changes -- maybe one click per pass -- until you get a feel for what works for you. The adjustables are great learning tools, and wonderful shavers. Enjoy!
Start off with a 3 setting. You can adjust up or down as you see fit. Your goal should be to get the best shave (i.e. closest and irritation-free) at the lowest setting you can.