Some folks have suggested the GH issue is well addressed, well, it does not matter a great deal as long as there is enough relative to NO3/PO4 etc.

Most folks have higher GH's, invariably someone mentions Mg++.
This is supplied in many trace mixes and plants need some, but not like many of the other nutrients. Most high Gh water alos has Mg, this is not 100% the case, but topping off with a 1 degree amount of SeaChem Eq or Ca/Cl2/MgSO4 will address most GH related potential issues so you can rule any of that out with one simple dosing after the water change.

Here are the links that support the ranges suggested and the evolution of where and who did what.
Here's several paper that detail the general concepts: some on plants alone, and then a few for algae and plants,

PMDD from Paul Seras and Kevin:
Control of Algae in Planted Aquaria

Note the low T12 lighting.
Also, Paul is careful not to suggest that the PO4 go to zero, or strongly limits plant growth. Some assume less = better, this is not true.
Also, you can find the infinite series dilution at the bottom, the same approach can be used for all nutrients, not just Fe.
Steve Dixon and myself started testing the hypothesis that Paul put forward in PMDD, and we showed it to be false and today PO4 is dosed in many routines.

The ppm's for EI are based off Gerloff, 1966 and critical concentrations, Liebig's law on the minimum.

http://www.new.aslo.org/lo/toc/vol_11/issue_4/0529.pdf

20-30ppm are the critical concentrations for aquatic plants.
See table 1 also, this is similar to the PMDD recipe.
The light in units of PAR is about 100 micromols/m^2/sec
So moderate light, not high or that low either.
PMDD ranges are more likely to be less for their light intensities used.

Liebig:
On the Origin of the Theory of Mineral Nutrition of Plants and the Law of the Minimum -- van der Ploeg et al. 63 (5): 1055 -- Soil Science Society of America Journal

As you can see, the credit is also given to another => Sprengel.

and Hoagland 1938(1933 for Gerloff's reference)

http://www.siu.edu/orda/igc/proceedings/01/larson.pdf

Some hornwort NO3 discussion

http://www.siu.edu/orda/igc/proceedings/01/larson.pdf

You can see hornwort removed about 100ppm in 10 days, about 10ppm per day. This is at full sun though.
Gerloff's test showed about 20-30ppm for NO3 to provide non limiting growth. This was grown at

The algae vs plant issue is well addressed in Bachmann et al, 2002:

http://fishweb.ifas.ufl.edu/Faculty%...macrophyte.pdf

and a more simple version of the article for the general public:

http://fishweb.ifas.ufl.edu/Faculty%...tics2004LR.pdf

Regards,
Tom Barr