battery capacity dropped down to 58% from 86% in a few days

Is this normal? And is there a way I could prevent this from happening when I get a new battery?

I've been using my macbook pro in a normal way, mostly for browsing and sometimes skype.

Usually I work on power, but sometimes not. At night I take it off the charger.

I calibrate the battery every 2-3 months.

I find it strange that after 311 loadcycles the battery capacity has dropped to 58%.