Hi BSFman, The 1st time I added some....I did the guy thing, I just poured some in...
In your bigger system, try starting with around a 1/4 of a cup.

When I do add now, I use about a cup...to 2,000 + of water. ( about twice a month)
I adjust that, accordig to how much of a plant load I have. Lots of young lettuce plants, seem to suck up the iron. It's pretty easy to notice the yellow leaves. After awhile of trial and error, I got a feel for it. I try to add just enough to stay ahead of the curve.

In addition, I use a liquid iron product from Lowes. I think the brand is Sunniland (?) A gallon is only like 5 or 6 dollars. It helps make the Maxi-crop ($) go further. Bannana peels are the only other thing I add on a regualr basis.