I use a "distanceBetween" node to have the distance between the focus_locator and the camera.
Use the connection editor to link the wordMatrix of the cameraShape to the inMatrix1 and worldMatrix of the locator to the inMatrix2 param of the distanceBetween node.
In second time use this mel line to link the distanceBetween and the manual focus distance (Be sure that Maxwell manual focus is enable before) :
connectAttr -f distanceBetween1.distance cameraShape.mxFocusDistance;
In fact I have done this once and I save ma camera rig in separate file to import each time I want.
_________________
-------------------------------------------
IN EFECTOhttp://www.inefecto.com