Forward lean can be measured (not as an angle) by placing the boot heel against a vertical surface and measuring from that vertical line to the back of the liner just above the shell. Many of the current crop of boots run in the 55- 65- 70mm measurement, but some are as high as 82mm.
A person with a skinny leg will work best (center up) in a boot with more forward lean, on the other hand the opposite is true for someone with a large calf. The result of being out of place will be fried quads and usually require a small step in order to release the uphill skis' uphill edge to start the new turn.
This Fore/aft balance issue is the most important part of the equation that you have to get right for each individual, and we are finding that most folks are out of place. The "right place" is only about 2mm wide(front to back) at the top of the boot shell and is usually only found by trial and error out on the hill, with small incremental changes. But when you do get it right you will (profoundly) know it.
Ultimately you are searching for a tibial net forward lean which brings you COM (which is further up the kinetic chain) over the boot sole/ski center.
Keep in mind, racers are motivated to find ways to pressure the tail of the ski (watch Bodie Miller ski) which most of us find to be a lot of extra work when out free skiing for 6 to 8 hrs at a time.
Edited by miketsc - 10/6/11 at 8:24am