For a low light tank, you would usually go with 1x/weekly dosage. It's not really the dosage that varies from non-co2 to co2 methods, it's the frequency. With a high light tank, you have to fertilize at least 3x/week to keep up, but with a lower light tank 1x/week can usually suffice. The other very large variable is the amount of plants you have and whether they are fast growers. What I have done in the past was to just dose 1x/week all nutrients as per the recommendations, and the day before I would dose again I would test nitrates. If they were at 0, I would assume that I wasn't adding enough and would start dosing 2x/week. If they were high, I would assume I was adding too much of everything and scale back the dosage a little. It's not one standard dosage that fits all tanks since the amount of plants and fish and lighting all plays into the fertilization needs. When I had a large amount of fast growing plants in my tank, I would have to dose 3 - 4x/week just to keep some nitrates in the tank. Now that I've pruned it back, 1x/week is fine.
I made up a fertilizer calculator to calculate how much of each thing you have to add to get x ppm in x gallons of water, if you want it it's in Excel format on my website,
http://beginneraquarist.petfish.net under fertilization I think...
When I run the numbers for 40 gallons of water, I get the following recommendations:
KNO3: add 2/3 tsp to get 15ppm nitrate (and 9.4ppm potassium)
KH2PO4: add 1/8 tsp to get abot 1.8ppm
K2SO4: add 3/4 tsp to get another 12ppm potassium (giving you a total of 22.4ppm)
Plantex CSM+B: add 1/8 tsp to get 0.25ppm of iron
As your plants recover and start growing you may have to increase this to 2x/week but just try 1x/week and see how things go.
If you're fighting bga, I would definitely recommend at least a 24 hour blackout to put a nice dent in the bga, followed by a 50% water change/gravel vac, repeating if necessary. This along with the correct fertilization and water changes should really turn things around. BGA will die quickly without both light and the organic nitrogenous compounds from the breakdown of excessive waste in the substrate.