I just measure a range over a number of lighting types from the distance from the bulb.

That way I know that a current 130W light at 10, 15, 20, 24" away will have say 300, 150, 50, 30 micromols no matter what gallon range I use.

T5's HQI and other light types are measured this way, and then I have a good idea.
Plants do pretty well down to about 20-30micromoles, 100 is pretty good for most.


Regards,
Tom Barr