Interesting discussion but I believe one should start with the geometry and dimmers before using height as a control.
At 20" above the water
and 120 degree lenses and considering each diodes is a " point" the light cone projected is 69" in diameter.
- Dimensions: 16x8.5x2.4inches. Item Weight: 6.8 lbs
- One light is ideal for 24”x24” coverage
- Max Coverage at 8" height: 30”x24”
- Core Coverage at 8" height: 24”x24”
- Avg. Power Draw: 105W±3%
- (55pcs) High Intensity 3W Bridgelux/Epileds LEDs
- Channel one: Violet(420nm), Royal Blue(450nm), Blue(470nm)
- Channel two: Green(520nm), Red(660nm), Neutral White(6500K), Cool White(12000K)
- Lens Angle: 90°
Just consider the edge diodes.
Say centers are 14" apart left right( long axis)
Your 40 is 36" 36-14=22"
Your starting height would be about 6" to cover the water surface with little spill outside the tank.
Then just dim accordingly.
Now that is the err " technical" way to look at it.
The practical advice above is valid but knowing the physics also helps
Now the problem is the quality of the lenses and how they are measured to determine the angle.
In a sense they appear " too good" in general meaning they concentrate too much light in under their rated angle acting like say a 90 is like a 60.
Assuming the 120's are more 90- like your height goes to 10" if the goal is to cover left to right and that is your "target" dimension.
Front/ back just becomes whatever it is.
At 120 ( not the corrected 120 to 90)
and 6" your front/back spread is about 26" on your 18" tank.
If you want to do it the old fashioned way just tape a bunch of paper together and draw a rectangle the size of the tank surface area.
Raise and lower the light (on of course and maybe just the white channel) and see what height fills it to the ends.
Want to go old fashioned and techy use a lux app to take your eyeballs out of the equation. Suggest just using the white channel to get relative intensities.
Lux is tailored to white light moreso than blue. Just will give higher readings so theoretically more accurate. $12 dedicated lux meter would be somewhat better.
Helps you to get a picture of your light field uniformity.
Fun calculator:
A simple calculator that determines spot size by a light's beam angle
store.marinebeam.com