I would think the manufacturers published chart is very accurate. No name companies that you have never heard of and who advertise for an extremely low price might be suspect, but a name brand simply has no incentive to misrepresent that chart. It might result in a single sale, but would sour that customer for future purchases.
SpiderFarmer is reputable and supports the forum. I had the same doubts initially, and I bought a PAR meter (Highly recommend BTW). I was able to determine that published charts exactly matched my readings (Disclaimer: other manufacturers, not
SpiderFarmer). A PAR meter is relatively cheap and is invaluable in setting height/dimming.
Personally, I set my lights up high (approx 3-4') and adjust the dimmers for the PAR that I want. Why do that when you could use less energy with the lamps closer to the plant? Penetration of the canopy and a less drastic fall off of PAR level from the top to the bottom of the plant. The inverse square law, as pointed out by @
tucume66, accurately predicts the level of fall off of intensity. So a light that is further away will 1) require more intensity, and 2) have less fall off than a light that is closer to to the plant.
That is why a light that outputs 1200 is useful even though you only need 600-700. In the outdoors, there is zero detectable fall off from the top of the plant to the bottom. That is because the sun is super bright and super far away (inverse square law).
Running lights at a brighter setting also means that you have to have better airflow to keep things cool, so make sure that you have the cooling capacity if you use this method.