The primer pump gets the system up to a specific pressure. As fuel runs out of the system, the pressure falls. Ideally, you'd test for 1 drip per second at full primer pressure. If you're not leaking fuel in a stream, the fuel pressure will stay sufficiently high at one drop per second. If you are not close to the proper setpoint (late), the fuel will run out of the drip tube and the pressure will fall quickly...........thereby making the check for one drop per second inaccurate.

Yes, the best accuracy is with full pump pressure..........but few will continue to pump the primer while simultaneously attempting to set the IP...........you'd need three hands.