First thing would be is that watts per a gallon mean nothing. It's an old out dated measurement that has been replaced by par light data. Par is the true effect light had on the plants. Many different lights even at the same watts rating can have higher or lower par values. It also makes a difference in the distance of your light to the top of the substrate.
From your original posts your having issues with algae. Depending on what algae you have in your case green spot, its to much light intensity, lacking nutrients or issues with co2. So let's look at what yoy have. I would say your tank light wise would be in the medium to high light range. What's the distance from the bottom of your light to the top of your substrate? 7 to 8 hours is fine for a photo period but remember if the plants are being driven by higher light amounts then you need more real nutrients. Gsa is normally an issue with low phosphates. But you also need potassium to use the phosphate looking at what you posted your dosing its obviously an issue of low nutrients. If you read that article you posted a link to there is zero confusion or conflicting info on fertilizer and dosing. Tom barr is one of the best on Ferts and his dosing approach of estimative index dosing solves tons of issues. It does this by giving the plants a bit more then they need. It's called non limiting nutrients being added to the water. This removes the issues of nutrient deficiency.
Here is what a dosing set for your 20 high would be like.
1/8 tsp KNO3 3x a week
1/32 tsp KH2PO4 3x a week
1/32 tsp K2SO4 3x a week
1/32 tsp (2ml) traces 3x a week
1/4 tsp KNO3 3x a week
1/16 tsp KH2PO4 3x a week
1/16 tsp K2SO4 3x a week
1/16 tsp (5ml) traces 3x a week
I would also dose seachem Excel in your tank as per their direction.
I think if you were to follow a program like this you would see major results in a week or 2.
Is also recommend reading a ton over at thus site on fertilizing and nutrients in a tank.
One of my several tanks is a 20 high planted I've got about the same amount of light you have. Once I set it up on this program my algae issues went away and my plants grow awesome.