I use Poser units. I put a ball where I want the camera to focus and note it's x, y and z coordinates as xb, yb and zb. Then I take the Dolly coords from the camera and let them equal xc, yc and zc. The focal distance is: sqrt((xc-xb)^2+(yc-by)^2+(zc-zb+1)^2). This seems to work well...now if I can just figure out how to get it into a python script!