PMDD dosing question

I've set up my planted tank after a hiatus of 3 years and have forgotten
some basic facts.

56 l tank
1W/ l lighting.
DIY CO2 achieving 20ppm.
moderately dense planting with Limnophila sessiflora and unknown (to me),
medium-sized Echinodorus.
dosing with 4 drops PMDD per day
laterite balls under the swords; coarse gravel substrate.

Plants pearling a lot and growing quite well.
A stable, tiny patch of cyanobacteria (on a plant stem at the surface right
under a light) and a bit more filamentous green algae than I'd like. Some
BBA fading away. No fish yet.

The NO3 and Fe remain unmeasurable after 2 weeks of this regimen. What
levels of both of these nutrients are desirable? Anyone want to take a stab
at an appropriate PMDD dosing rate?

Thanks in advance,
Nick.