Thanks for the approximation!
I don't have access to a par meter so an approximation using the watts/gallon rule is pretty useful, when selecting/building my unit
I've read off the internet that's it's as high as 1:4 led watts : t5ho watts to 1:2 ratio
Here's one of those articles (http://www.americanaquariumproducts....hting.html#led
but I think they're also a manufacture or a distributor so it's kinda biased, but a pretty good read.
You have to be careful when applying W/gal or approximations, they can be way off with LED's and other different brands, and distance seems more important with LED's vs say T5.
The factor you list for a ratio above accurately describes the problem.
So if the light has a specific PAR and curve, then you can use that for estimation, but the problem is when folks assume ALL LED'S are the same, or all T5's are the same etc, and folks sure as heck sure love to do that type of assuming as much as they love chocolate.
Happens with EI/fert routines, CO2 and of course.......lighting.
So do not assume, verify.
That's why all the whacky charts, Calculators and graphs etc.......will never replace a PAR meter. I've argued this point for a good 15 years and it's held up pretty well, and I have had some pretty sharp folks nag me on this one.
The problem is that brands do not stick around for lomng, they come out with new models and new bulbs and new color temps and new reflectors, and new ballast types.
It's like hitting a moving target.
So the PAR meter can test each new one that comes along etc.
We cannot assume that every T5 and every LED model from here till the cows come home ....will be the same, even within say 20% error.