I don't really know about such things but I've tried Ray's calculator for you and that comes out with 53 teaspoons per gallon, which is a bit more than 3 times what you had calculated..
Ray recomends 125ppm which comes out at 66 teaspoons per gallon.
Take a look at Ray's calculator
here. You enter the ratio (1:100), the target ppm and the Nitrogen figure (15 in your fertiliser). It's the calculator at the bottom of the page I've linked to rather than the other one which is for manual mixing.