The EI dose for nitrogen is actually 7.5ppm per dose. Dosed 3 times per week. The calculations below are based on that. You can change them for 10ppm if you like. The results from YANC are closer than your formula.
is a good explanation of the math behind the calculations.
The formula is,
Desired ppm / % of element in fertilizer x 75 (75 is the constant for ounces per gallon)
7.5 ppm / 1.5% (nitrogen) x 75 = 0.0666 ounces per gallon
0.0666 x 40 (gallons) = 2.664 ounces
Convert ounces to milliliters,
2.664 * 29.57 = 78.77448 ml
Since we want 7.5 ppm of nitrate NOT nitrogen we convert that dose to nitrate. The reason for this conversion is that nitrate molecule weighs 62 grams per mole; the nitrogen content of nitrate is 22.5% of the total weight of the molecule. So we convert it like this...
78.77448 * 0.225 = 17.724258 ml
So the dose is roughly 17.7 ml to raise NO3 levels to 7.5ppm in 40 gallons.
So that said you can do the math yourself or use the YANC. Calculators are easier. Yet that is the math behind the curtain lol.