How much line?

Manufacturers generally recommend filling the spool up to 1/8" from the edge of the flange. That's playing it safe. If you can control the speed, there's quite a bit more distance to be had by filling your spool closer to the edge.
                                                       Way to go, markG!   

More distance yet!

As I'm shorebound most of the time, anything to improve my casting reach is a gift from heaven. If your reel has a disengaging line guide, centering it before you cast will give a noticeable improvement.  With a wider spool, the increase is even greater.

Thanks, Sandman!