Technically, no, in either case The base concentration of the fertilizer was 21-5-20. So of that teaspoon, 21% was nitrogen, 5% phosphoric acid equivalency, and 20% potassium.

I'd need to know the exact weights for exact dilutions, but 1 tsp into 1 gallon isn't much. Figuring it as sugar, 1 tsp = 4.2 grams. Which goes into 3785 grams (near enough). So the translation is 4.2/3785, or 0.0011 (near enough).

So your fertilizer becomes 0.023 - 0.006 - 0.022. Near enough.

Half that if you use a half teaspoon.

To be clear. I weighed 10 teaspoons of fertilizer as my scale isn't that sensitive. This weighed about 43 grams (weighing several times gave results of 40 gms ~ 45 gms. So average 4.3gms/3785gms=0.001136. Now, 21% is N or 0.21*0.001136=0.00023857ppm N?

So it's actually almost exactly the weight of sugar (4.3 grams per teaspoon) and my numbers were pretty good. Chalk one up for off the cuff estimates. :-)

The 0.21*0.001136 would be a proportion, so you'd multiply by a million for the PPM.

The solution is 0.00023857 nitrogen fertilizer based, or 0.02 if you're putting this on a label. Which is 239 PPM nitrogen if I slipped the digits correctly. That works in my head, it's about two-one hundredths of a percent and very reasonable for most plants for the occasional to weekly feeding depending on the plant (my desert roses would get more than this with every feeding during the summer, but they're oddballs in the succulent family).

If you're doing the half teaspoon to try to target closer to 100 PPM, you were doing the right thing; the resulting solution is around 120 PPM nitrogen, 30 PPM phosphoric acid equivalent, 100 PPM elemental potassium equivalent (there's an 83 percent slip in there).