Milestones like hits, HR's, RBI's, etc are easy to shoot for because all it requires is counting.
Retiring just in time to preserve a .300 batting average is much more complex, and I seriously doubt Pujols is sophisticated enough to plan for that. This is not some guy who runs Microsoft Excel in the clubhouse forecasting how much more he can suck so people will have a higher opinion of him when he retires.
If anything, he's proud enough of a guy to announce his intent a year in advance so he can have a season-long farewell tour, much like Jeter did.