Lucas formula is from the 70's. It's not for soil generally, but you can modify it. It's antiquated and outdated. Based on old growing philosophies.
It still works because it's based on the fundamentals of growing anything, and you can get a harvest from it,
but most of the time you have to supplement nitrogen anyways, so it's not entirely fool proof. The whole idea is
to save a measly $20-$25 every grow cycle...
The thre numbers on the front of the bottle, 20-20-20, is the amount of available Nitrogen, phosphorus and potassium respectively.
No mater how much water you use, that's how much of each is gonna be in there.