UMOL and LED Height

I would start here:
Inverse Square Law Formula

You could also use a light meter app (or an actual light meter)

 

Firstly, if you hung your light in a dark room at 18" from the floor, and then turned it on and measured the area of the spot on the floor that was illuminated, you could find how many μmols your light emits in total. You'd divide 1203PPFD by the area you measured in ㎡, and then youd arrive at total μmol/s output of your fixture.

Secondly, you'd have to expirementally calculate your beam angle. This means measuring the width and length (in meters) of the spot on the floor that's being illuminated, while you hang from different heights. Once you determine a correlation, ie 1in up from 18" equals "X" more to the width, and "Y" more to the length, you can then estimate the length and widths at various heights (using the multiplication factors you found), at that point you can then calculate the area from the estimated width and length (in meters), and then divide your total μmol output from step 1 by your estimated/calculated area (in ㎡) at the various heights from step 2, in order to determine the various PPFDs at the heights you're curious about.

Whose to say how accurate the 1203PPFD figure they gave was, or if it was an average PPFD or a hotspot PPFD. The figures you calculate will be averages, which means that it will be brighter than your calculated intensity in the middle, and dimmer than your calculated intensity at the edges. This technique should give you a good idea but it's a bit lengthy. If you know how your fixture spreads light already, and don't need to expirementally find that out, then it will be easier. LEDs only shine light on 1 side of the chip whereas an incadensect shines in a spherical pattern. This means that the inverse square relationship normally present with a spherical emission will not be valid with an LED. It will decay at a different rate. I wish it were as easy as dividing PPFD by hang height, but unfortunately it's a bit more complicated than that. There will be some variable of error as the multiplication factors will skew with height as light emits in a circular pattern and your fixture is most likely rectangular in shape, ie the closer to the ground the more rectangular the beam spot, and the higher up the more rounded the beam spot.

 

That's why a $30 LUX meter is worth its weight in gold.

 

Yeah a LUX meter might not give you PAR readings but for the purpose of dialing in lights it does its job well

 

I suppose youre doing this in order to figure out hanging height? Its better not trying to do hanging height just by numbers. Hang it high and bring it closer little by little looking always ar new growth checking for signs of unhappy plant. Pale green and purple stems and petioles, poor plant stance etc. If that happens just raise a bit, raise temps maybenmaybe adjust nutes a bit