EI worked at 5.5 watts per gal over a 20 Gal 45cm deep tank, or about 450 micromols.
Most weeds are around 400-600micromols of light saturation.
You get higher light use efficacy at lower ranges though.
My rational was if the light and CO2 are both non limiting, what nutrient demand would this equal? About 100% uptake vai EI. Since most folks use less than 450 micromols, EI should cover anyone from being limited. ADA AS and DIY Soil methods do this also, they just add all the ferts in the sediment, but the end result is similar. 6wpg is rather vague, it could be off by 300-400%, maybe more depending on which light set up you may have. Hence the use of a light PAR meter. I do not suggest folks use this much light, never have actually, more in the 2w/gal range for most, a little less for T5's even.
Regards,
Tom Barr