If the injectors had no dead time, the PW would be proportional to lambda (L).
From what I had understood, for some (MAP, RPM), have 2 VE values to get (PW1,L1) and (PW2,L2). Then draw the line trough this 2 points. When that line cross L=0, you get the dead time.
This is a bit theoretical, since this is done for one (MAP,RPM), you should have to take the mean, take care of Voltage, the chosen VE values.
Better to make an Excel sheet or other.