Originally posted by DWS1 Don't have the 150-450, but on my 300 I determined (close enough anyway) the center of gravity by attaching it to the camera, setting it on a flat surface with a large dowel under the foot. Then rolled the camera and lens slowly on the dowel and determined the point where it tipped from camera down to front element down. Marked that point on the foot and figured out how long of plate I needed to get that point over the tripod center. I suppose you could do the same at 150 and 450. Then figure out how long of plate you need to get those two points centered on the tripod. It's not exact but better than eye balling it.
The issue is on many lenses, like my DA560 and Adaptall 200-500/5.6 is that the foot design is a) not long enough, and b) in the wrong place, to do this. In both the cases, the CofG is beyond the foot. On the DA560 I actually think the designers simply screwed up, because the lens would be much more balanced if the foot was mounted in the opposite direction.
Note, on my Sigma APO 70-200/2.8 EX the foot can be reversed and this is useful in achieving balance with and without teleconverter attached.