Here are the results of my observations

The following maths treat all cylinders as a single measurement. Figures will be 4 times higher than the real value for a single injector.

WARNING: PEDANTIC GUY AHEAD

Tank 1 (country):

- predicted 28405mL

- actually 28360mL (error: 0.15%)

- PWM fired 332468 times for 2195.598 seconds

- average pulse width: 6603uS

- average fuel quantity injected: 85.301uL

Tank 2 (city):

- predicted 30954mL

- actually 30950mL (error: close enough)

- PWM fired 385078 times for 2403.540 seconds

- average pulse width: 6241uS

- average fuel quantity injected: 80.373uL

The difference in pulse widths is 362uS and the difference in fuel injected is 4.928uL. From this we can infer that the fuel pumping rate is 0.01361325 uL per uS. Converting units, that's 204cc per minute which is in the expected ballpark for a small car.

The total time to transmit the average injected fuel amount into the cylinder is then 6266uS (tank 1) or 5904uS (tank 2). This predicts the injector time delay at 337uS for both, depending on how inaccurate those particular fuel pumps were.

Thanks dcb and everyone else for all the advice!