Every spec has a tolerance. In this case the tolerance depends on your weight and how you're riding. If you were mountain biking on the thing I definitely wouldn't go above the line. But at your weight, and given how you say you're going to ride the bike, I think you'll be fine. My commuter has a rather short post that I run in a similar position and I've never had an issue. But it's also a heavy, straight gauge, hi-ten frame.

The point is that it's not an exact science and folks who live and die by the rules simply don't know when it's ok to bend them.