Check out this on -NPK ratios- Canadian University study

[Tomatoesonly]

Yes, that's how I understand it. Monocot and Dicot are the general solutions and for canna they have a more specific one.

Exactly. You can also simply divide by 4 and then multiply by the desired value if you want to make it very simple.
If 4 is 100%, you can of course also simply calculate 75% for 3.

Holy cow man, it's a good thing you got me straightened out. I was all kinds of wrong.