When I was working out where the engines would go on my STOVL Canberra, I figured that when the nozzles are swiveled downwards for vertical take-off, the center of the four nozzles* should be in line with the load cg on the aircraft.
* the center being where two lines cross, one line running from front-right nozzle to rear-left, the other line running from front-left to rear-right.