If I were to add up all of the milli-seconds of on time for the fuel injector, would I come close to fuel consumption? This would probably assume that the fuel pressure is constant..
EFI is based on the premise that the fuel quantity injected is directly proportional to the spray time.
However, the injector takes a finite amount of time to open and close, during which no fuel is injected.
Therefore, the electrical pulse width is equal to the desired spray time + dead time.
Obviously, the injector is inductive. When voltage is applied, it takes some time for the current (and thus magnetic force) to overcome the spring preload, and therefore move the needle away from its seat.
In the ECU, the dead time calculation can be implemented as a 1-D lookup table as a function of battery voltage, or even as a y = ax + b function.
Dead time is on the order of ~1 ms, so at low loads it becomes an important factor.
Fuel pressure across the injector should be constant for most setups, notwithstanding silly GM 3800 fuel systems and the like.
Fuel flow is related to the square root of the pressure difference, so it's not a huge variable anyway.