Don't think about it, just do it.

[Irulan]

if you go too far back, you unweight the front end which is not a good thing. The "chest on the saddle" stuff is incorrect. It's all about where the center of mass (COM) is, in relation to the pull of gravity.
Look at it this way - you can draw a vertical line through my hips and feet to the ground... that's my center of mass, over my feet and pedals. If I were further back, my hips would not be over my feet, they'd be over the rear hub, and my chest would be above my feet, adn that is not a well balanced stance. You WANT your COM over your feet perpendicular to the ground.