With your fluke dialled in to measure DC voltage, measure your battery voltage (V) with your engine running above 80 deg C.
Next, take the cap off the diagnostic port X11 (the round capped connector on the injection module).
Put the black probe of the meter to ground or battery negative and pit the red probe into pin 3 of the X11 port.
You should get a voltage (X) reading.
The duty cycle (%) is (X/V) x 100.
Eg. if your battery voltage is 13.8V (V) and the voltage at pin 3 (X) is 5.5V, then the duty cycle is (5.5/13.8) x 100 = 39.8% (slightly rich).