Some Barners can give a straight answer.

The amount per tank is irrelevant, unless you go to wild extremes. All that matters is how much of the actual herbicide you put out per acre. Ben gave one formula using Milestone, and there are others here that use 2-4DB. Pick one and get that amount on your plot on a per acre basis. Nobody can tell you how much to put in the tank without knowing the variables, so only you can figure that.

Gotta start by figuring the exact size of your plot. Measure it and figure sq ft, then convert that to acres. Then you can multiply that number by the numbers in your formula and determine exactly how much herbicide you need on the field.

Next, you have to figure out how much herbicide your sprayer puts out. That will be determined by the sprayer and the speed that you drive. Since you are dealing with a small plot of less than an acre, you could just do a test plot. Say it is exactly a half acre - lay out a plot in a pasture that is exactly a half acre, fill your sprayer with water, then spray the water over the test plot being careful to run at the same speed you want to use to spray. Then see how much water it took and adjust the numbers to fit your recipe.

This takes a little time, but it's essential to calibrate your sprayer. Good luck