The original OP is asking about light height on a simple softy tank, sending him off chasing numbers and potentially wrong numbers for what?
Interesting take...
I'm actually doing this and have been for a little while now (not saying forever), as are others, and it does work....I'm not just spouting theory. If you catch me in the photography forum with any of this...or saying much of anything that doesn't end with a "?" - then I'd be spouting theory!!
Are you really saying the OP is better off guessing?
You really should read through some of the threads marked with the lux meter tags (or simply search on the forum) and see what you are missing.
Measure PAR underwater with a $3000 meter if you want - Dana Riddle does, so you even have a fine example to follow. (He's written about lux meters in his write-ups a few times too I think.)
But that's expensive, a hassle, and totally unnecessary for this purpose.
Makes no sense unless you have those meters laying around like Dana!! (I wouldn't use a $15 lux meter in that case either! But I would still convert to lux for the forum....cuz who the heck can afford a PAR meter!!!)
What I am interested in is a fellow hobbyist asking questions about lighting like @FilterFreak who might think or get the idea that lux meters are tools they can't or shouldn't use (or even think there are no tools!!) because someone online says that guessing or taking someone else's light settings from across the internet is better for some crazy reason.
If you want something to trade with people online, it's your lux readings from the water surface. Why? Because that reading will be universal for everyone - and that's a meter everyone can afford.
tells him nothing about the actual readings in his tank at any given level taking actual depth, water clarity, surface agitation and shading into consideration..
I challenge you to take all that into consideration.
Then I challenge you to explain your findings to me and
@FilterFreak in a way that's useful to us in setting up our lights.
...since our water conditions aren't the same as yours.
On the other hand, if you tell us the peak lux reading you get from your tank - which couldn't be simpler for you to get - both of us have a great shot of duplicating that lighting level (as closely as needed - say to within +/- 2500 lux) using our own meter.