Size Of Imager Impacts Field Of View?

There is at least one online calculator that includes height calculations: This one by JVSG, who also make a design application for Windows that will help with more complex designs: IP Video System Design Tool

However, as John's link shows, the "perfect" height will depend on a lot of factors, including the view you need. Put it too low, you may not cover as much area; put it too high, you'll only be looking at the tops of things. A program that can give you simulated views, like the one above, will help... experience will help as well. Sometimes you need to just get up on a ladder or lift, and see for yourself what the picture will look like from various heights.