We only use a gallon at a time (and mix additional gallons if we need them).
Our backyard plot is approx. 20' x 40' and, if I'm foliar feeding (using a fine mist), I can do the entire plot with one gallon, plus putting a little bit on the foliage of our small peach tree.
If, however, I am using the Garrett Juice as a soil drench (which I don't do very often because of expense) your calculation is about right -- 200 sq. ft.
I hope that helps. Happy gardening!
**Take time to stop and smell the flowers
(or... as my ladybug refrigerator magnet says
"take time to stop and eat