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Considering that most down light set ups will use spots, I figured I should come up with a better multiplier than the generic '1.5'.
It seems that the multiplier varies according to distance from the target. I'm not sure why that would be the case, as the target was always inside the cone of the 'angle start' setting. Anyway, the variance is small so a new 'generic' value could apply.
I found that at 1m, the multiplier is 1.18. At 2m it's 1.2. At 2.8m it was 1.22, and from 3m to 8m (as far as I went) it's 1.24. I think a generic value of 1.22 will give a decent result in most cases.
I then returned to the street scene from my tutorial, set the lights up exactly to measure 8m directly above the grey card, and 1.24 was the correct multiplier. Curious, I went back to the apartment scene (where 1.32 was the multiplier) and can only assume that the difference was because the grey card was not directly below any particular light (it was in front of the TV). I moved a light to be exactly over the card and exactly 2.8m above, and a multiplier of 1.22 was correct.
The problem of multiple overlapping lights still remains. There can be no accurate process for this as it depends entirely on the angle start/end settings, the height of the lights, reflectance of nearby surfaces and how close the lights are to each other. In the case of the apartment room the grey card had one solid light and 2 significant partial overlaps of the angle start light pools. If I took the base calculation for the light (98%), multiplied it by 1.22 (119.56, rounded to 120) and divide by 2.5 I get 48%. That figure just with the 3 lights in play was still too dark (RGB 111) so it shows the contribution of the other 9 lights in the room. If all were set to 48 the grey card would be bright, but still well inside an acceptable range.