feeding too much??

I've got two fantail goldfish, have had them for about a month. I've always
been a little confused about the feeding thing. I've read many places that you
feed them just as much as they can eat in 3-5 minutes. Well, the flake food
they can scarf down really fast, so I seem to give them a LOT of it. Two big
pinchfuls. I have some granular food too, and one little nugget takes them
forever to eat. They can chew on one piece and spit it out and back in again
for a long time. So I'm not quite sure how much of that to give.

Let me back up a little so I won't confuse you. I feed them the flake in the
am and pm and a little nugget food snack at lunchtime. Yes, my fish are
growing quite a bit. They seem happy but always hungry. Every time they see
me they swim to the front of the tank and to the top, the little pigs. LOL!

I'm afraid of either starving them or feeding them too much, like Dr. Suess's
"A Fish out of Water".

TIA,
Stephanie