I use the same numbers as cyben during recruiting (0, 10, and 20). I actually put all the recruits in a spreadsheet and subtract 10 from the red and add 10 to the green, then evaluate on the modified ratings.
The actual potential ranges are:
Low: 0-5
Avg: 6-20
High: 21+
Those are the maximum improvements, so a low potential rating can never go up more that 5 points.
Now, how much improvement you will actually get depends on practice minutes, WE, and playing time. A guy with high potential but low WE may only improve 15 points, but you can mitigate this somewhat with more practice in cores and more playing time.
IMO, playing time is critical for low-WE guys, because it increases the rate of improvements, and also improves WE. I will recruit guys with WE in the teens, but I will try to start them as much as possible from the beginning.