I think you have a slight misunderstanding of how "Luminaries" perform compared to the individual LED chips that the Luminaire uses. Depending on the number of chips used, what current they are run at, the spacing between the chips, the thermal characteristics of the fixture itself, and if any optics are used i.e. lenses/reflectors; there can be such a wide range of output. Even the angle of the reflector would play a big part in system performance.
Now you could take data sheet info and then guess and extrapolate the rest of the information... Or you could skip the theorized calculation step and go right to measuring and logging real data. There are a whole lot of claims about watts consumed, efficiency, and performance on this forum that I question unless I saw pictures proving otherwise. It's one thing to work something out on paper, and it's another thing to put it into practice and measure the results.