Hmm true, I has heard that formula is total BS and does not work, however i am going to give it a shot.
But yes I think it was specificied as you said that the forumla is even less accurate if not complete inaccurate when we start talking about custom light spectrums
They're
all inaccurate - the question is how inaccurate, how does the inaccuracy change over time, and, does it matter that they're in accurate.
To my way of thinking, it really doesn't matter.
How's that?
The main reason to use a meter (of any kind) is to get getting your light levels "in the ballpark". Unless you're pushing your plants to the light saturation point, even being 100µmols out isn't a big deal. You can pretty much set it and forget it even if you're only at 700µmols, for example. Cannabis will do just fine at that light level.
On the other end of the spectrum
if you want to get the most out of your grow, you'll be going for a PPFD of 900 to 1000 µmols. At that level, it's
not wise to set it and forget it regardless of how you determined your light levels.
While it's interesting to some growers (me being one) to know "exactly" what the light levels are, the important point is to adjust the light levels so that the plants aren't getting too much light. Once you know that your plants are being all they can be, when your light meter reads 57,750 lux, for example, you've got your magic number. After that, the key is to make sure that your meter is consistent, regardless if it's a PAR meter or a light meter.
In short, if you're a high light grower, a PAR meter or using the exact right formula will get you to the starting level more quickly but you still have to dial it in to find the light saturation point for that particular grow.
