So 2.9W/gal for 4x 40 W T12's on a 55 gal tank is still lower light than 40micromols........
So around 3W/gal is considered high light by many that use the conversion, then they run out and apply it to a T5 or a PC light set up and assume they are the same.
I'm not sure why testing is so horrible for lighting when so many on the forums over the years harass people about the need to test things like NO3 or PO4 critically, or Fe and Mg etc......balancing K+ and all sorts of hoodoo.
Then do not measure light.
CO2 has it's many issues, but light is fairly straight forward and not hard if you use a meter. Hoppy's graphs illustrate the point pretty clearly.
Does not matter what differences tank to tank we might have, with a PAR meter, you have a good idea for comparisons. Quick, easy and lightly(pun intended) do not have to do it again from then on. Cost is the main issue for the PAR meter, some have suggested renting, but many are too cheap even for that, but are willing to spend $$$ for test kits.
You can also buy it, use it, rent amongst local folks, then sell it.
Why would you need it much more unless you are like Hoppy or myself interested in doing comparisons in real or hypothetical cases?
Might as well sell it back for 200$ or so.
If you buy it for 200-240 in a group buy, it cost you next to nothing.
Folks will buy them for the 200$ range pretty easily.