Thinning in the middle of hairline + Norwood Help

[nyjrb]

I've noticed that along with my temples, the center of my hairline also seems to be thinning. Is that common? How does that pattern usually end up? Any possible way of knowing where my hair loss might stop? Whenever I see receding hairlines, the temples go way back but the center still has hair. When does crown thinning start to happen?
Also where do I fall on the norwood chart? Last month some said a thinning norwood one, now someone said a NW2. How could I go from a NW1 to a NW2 in a month?!