A procedure that can work fairly well if all you have is lumens:

First convert to LUX:

https://www.rapidtables.com/calc/light/lumen-to-lux-calculator.html
Then LUX to PPFD:

https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator
30" lists 4000 Lumens.

Footprint of light panel is roughly 28" x 4".

Beam angle is 120 degrees.

Use 12"..

Area @ 12" is 21 + 28 + 21 X 21 +4 + 21" = 22.3 sq ft in free air..

Translates to 1930 LUX.

25.47 "PAR" (PPFD)

14361 LUX . At the surface of a 30x 12" (3sq ft) tank

192.54 "PAR"....

Height would be 2.3" off that "surface" (3 sq feet) ignoring the spill at the 38" l/right.

So you now have a surface "par" and a free air at 12" "par" measurement.

Now the tricky part..

**How much does water/glass reflection/refraction @ 12" "train" the light into the tank?**
From the above one could assume 4x increase in PAR due to the physics..

**101.88 "PAR" at 12"**

Listed = 111
48" Finnex planted plus 12" 107 PAR.

35.6W or 3204 Lumens (approx 90L/watt, current standard)

Possibly low.. but Finnex AFAICT doesn't list lumens.

Footprint 45x 2

So place 3" off the water line so we are estimating at 9" in the tank (3 out)

sq ft at 3" = 5.2 + 2 + 5.2 X 5.2 + 45 + 5.2 = 4.77 sq ft

7338 LUX

98.38 "PAR"

20.8 + 45 + 20.8 X 20.8 + 2 + 20.8 =

**Comes to 22.5 "PAR" at 12"..**

22.5 x 4 = 90

"Actual" = 107
Coincidence???

Anyone want to verify?

Just calculate the LUX based on surface area and lumens at a known PAR height.

Run it through the 2 above listed calculators and multiply by 4..