Jose,
You can't measure constantly any values here so you need a trigger to measure fuel dose at the injection pump. That's why you need rpm signal. It's a trigger for the moment that you should look into other signals from the IP.
That's an example of fuel dose. Rack position is combined with an ELR offset and that gives you exact fuel dosage. Measuring those two values at the moment of trigger signal (rpm) gives you the amount of fuel per one rpm. Changing this to time of injection pulse gives you proper signal for MPGuino.
So you don't need any pump specific volumes. You just need to calibrate it properly and voila!