as a very basic rule we use Watts per gallon (WPG)
anything at, or under 1wpg is classed as low light, 1.5wpg medium, and 2wpg is high.
It is hard to calculate as there is different diameter tubes, whic throws it off by quite a lot.
To get an accurate measurment you really need to use a PAR meter.
take a look here:
viewtopic.php?f=34&t=2271