What is the optimum throw distance?

I would say filling the screen is #1---Then you might need to zoom out for some material(that would be some of the quasi hd movies)to fill the screen. So, you need to be able to "over-fill" the 16x9 in order to be able to "zoom out to fill" 4x3 material.

Other things to consider are: angle, cabling, heat, noise and cone.

Optics and Geometry will be best near the center of the range (14ft). Keep in mind this is usually the distance from the screen to the front of the lens so mounting will be somewhat further than that. Lens is typically offset left or right so you need to compensate for that too.

Some projectors have offset too (distance from top of screen to lens height). Cheaper projectors usually have a fixed offset like 2". If you have zero offset than the projector lens center should be even with the top of your screen area. Sometimes projectors have offset that increases as you move the projector back. This is usually desirable because you can get the projector closer to the ceiling. Keystone adjustments can compensate for projector placement if your projector has them. I try and mount projectors as perfect as I can and then use adjustments to fine tune.

When calculating screen and projector positions I usually start at 36" off the floor for the bottom of the screen area. Then depending on screen size I now know my top of screen height. Add your projector offset and go from there. Depending on ceiling height you may need to adjust screen size or height off the floor to get things to fit within your requirements.