Guess what? I found someone with a PAR meter who was willing to lend it to me! The person has asked me not to reveal his identity so I can only show some results instead
About 1 inch from an LED and it reads

Massive!!
But once we reach the water's surface (still in air)

Dropped quite a bit.
And just below the water's surface

Dropped some more.
At this level, Ludwigia inclinata verticillata var Cuba is starting to turn a bit red in the veins.

That is quite a lot of PAR.
Here's where Ludwigia brevipes is starting to turn orangish.
Right in front of the glass.

Quite a lot of PAR! Too much I think, Tom Barr measured a tank with a full HC carpet and it only had about 50 umols
At the front too but on the right side where the hairgrass is growing much nicer and thicker.

Only 2/3rds of the PAR in the middle. Lush hairgrass might have to do with better CO2 distribution perhaps.
At the back, middle height..

So low! Now this I didn't expect. Now I know why the stems behind were slower than the ones in the middle. As a result of this, I have raised my lighting by another 10cm to get a better spread. PAR reading around 90 umols with the substrate still around 90 umols. Will try this configuration and see.
I've always had this feeling the soil is really low on nutrients, so I got some Osmocote.
And turned them into root tabs. Inserted 4 of them around the stems, let's see what happens in the next few weeks.
