Calculating amount of N, P, etc for Miracle Gro

Friend gave me six unopened 5-lb boxes of Miracle Gro 24-8-16. The blue powder stuff.

The label says 1.5 lbs feeds 600 square feet with the hose end sprayer (I have one).

Thing is, that seems pretty imprecise to me.

So if I’m trying to figure out how much I really need, I think I have to work backwards, right?

So if I determine a particular crop needs 0.3 lb of N at a given moment, I need 1.25 lbs (at 24% N) of it to get there. I assume I need to literally measure out 1.25 pounds of the powder, and then, figure out how much volume that is to determine how much water to add to it. Or just drop that amount in the sprayer until it’s depleted. Right?

And I guess the one concern I have, is if I’m using the sprayer how do I know I’m really distributing it it evenly among the plants? It’s not like I’m standing there moving down each row in a perfect, even speed.