Remember that calculations of the inverse square law must be modified slightly when you have multiple light sources. It’s not as straightforward as a quarter of the “power” for each doubling of distance from a single point source, especially when you have thousands of point sources, such as builds using strips or QBs.
You waste much of the energy of photons due to absorption when they hit any surface other than a leaf. Quite a bit, in fact, which is why you run strips low to the canopy and try to minimize wall losses, which can be 10-20% or more. If we’re OK with losing that much to walls, or if we’re OK with losing efficiency in using budget drivers that operate at 3, 5, 10, or 20% less efficiency than the best in class drivers, why do we care about using efficient LEDs? Why not just go back to bulbs, if you’re prepared to accept those losses?
I would still recommend around 25w/sqft of current high efficiency white midpower LEDs run at or near nominal current (July 2020), since most strips in this category are going to give you a lot of photons per joule in a good spectrum. As you mentioned, @Ginger_Rick, Bridgelux is the current king of the hill in a cost/benefit calc. They’re testing really, really well for the price.
Also keep in mind that the nebulous conventional wisdom of “penetration” is also modified not just by the necessary changes to the inverse square calculations when using multi-point light sources, but also by specific parts of the photosynthetically active spectrum. It’s now known that green, yellow and far red spectrum photons can travel through multiple leaves. This is a double-edged sword due to certain plant species’ response to various ratios of these wavelengths to each other, e.g. far red shade avoidance, but is generally a good bit of info for the conscientious DIY strip builder.