I don't pay attention to guage markings on the stripper, I just start from a too wide opening and keep narrowing until it strips well, that is, the largest opening that strips that specific wire.
Also, I prefer strippers that have guage adjustments rather than having to choose the right size each time. That's because I typically work with one wire size at a time (solderless breadboard, wirewrap wire, etc).
Am very happy with these two
http://amzn.com/B00004T7WR // for general wires, cheapo but works remarkably well
http://amzn.com/B005EXNWLM // for thinner wires
For teflon I am using a Teledyne thermal wire stripper but the first one above should also work well.