if the convergence point would be 100000m behind the flashlight just the divergence would be really low about 0.000001mrad if the lens diameter is 10cm
lets say at 1m I got 100cd
now I got to to 100.000m then my spot is exactly 20cm
so at 100km of the flashlight I get 25cd
if I calculate that lux to 1m I get 25,000,000,000cd
if I measure at 300.000km or one light second
I get a spot of 600m
0,0000027cd
If I calculate that to 1m
I get 250,000,000,000cd
so for this almost perfect thrower the distance to get an accurate reading would be a couple of light seconds away
there will be a point where the cd doesnt raise anymore
how far that is away depends on the divergence and lens diameter
as flashlights are really bad on divergence that point is really close to the light 5-10m should be fine
just go further out and when the calculated CD value dont increese anymore the reading is accurate
you are just not understanding the difference between a flashlight and a laser
take a laser pointer with 3mm beam and 0.25mrad single Mode diode
I measure at 0m 10000cd
at 1m 3mm+(0,25mrad*1m)=3,25mm spot
measured 8521cd
at 2m 3mm+/0,25mradx2m)=3,50mm spot
measured 7346cd
if I calculate that to 1m I would get 29384cd
at 100m 3mm+(0,25mrad*100m)=28mm
measured 115cd
if I calculate that to 1m I would get 1,150,000cd
at 100km 3mm+(0,25mrad*100,000m)=25003mm
measured 0,00144cd
if I calculate that to 1m I would get 1,440,000cd
for a Laser that will be in the range of a couple hundred meters to kilometers
it all depends on ratio for divergence and lens diameter
I got 15 years experience with lasers also calculating things trust me