I'm commuting four miles wth a few slight hills here and there and am really enjoying it, but my lower back gets a little sore.

It could be from carrying a backpack, but I'm not sure. I'm also about twenty pounds overweight, so that probably has a lot to do with it.

It's a very slight herniated disc thing, I'm fairly sure, and it's only a slight nag in my lower right back (actually the top of my butt).

Anyway, how high should my seat be? The bike is sized correctly (LBS), and it's new. So is there a way for me to know the right level to adjust it?

Thanks