It is an approximation, the opening takes longer than the closing, and is also dependent on vehicle voltage. But it is measured in terms of consistency between refills at the pump (factoring in refill technique of course). My saturn comes in within 1% consistently, your mileage may vary.
There was a proposal, assuming that your tank refill is accurate, that by keeping track of the number of injector pulses for the tank in addition to the open time , and keeping track of the actual refill amounts. That over several tanks you can find a value for injdelay and microsecondspergallon that best fit all the refills.
Of course if your refills are already within 1%, or off by a consistent percentage, then you might be like "why bother"
But if not then this is something to investigate.