it is irrelevant where the imagine source of the flashlight is
you get only higher lumens if you go to a negative distance towards the spot where the light should for a focal point,
but that is physically impossible
if the light has 20mrad divergence it doesnt matter where you put a ideal luxmeter as long as the beam is uniform in brightness,
as a reflector light is not uniform in brightness at close distances, so you have to go further away,
if you are too close you measure too low lumens in the donut hole or too high lumens in the ring where the brightness is concentrated
on a reflector light the spill also affects the measured lux if you are very close
the next factor is that your measurement would need to be taken on an indefinate small point in the middle of the beam,
so the bigger the spot is the less you get a false reading as your luxmeter has a quite big spot to catch light at low distances