Something you can do now is to fill up the tank with water and pretend your spray herbicide. Keep your ATV at 5mph or so. See how wide it sprays. IF your just using a wand, you can swap out the cone sprayer tip for a 130 deg wide angle tip. I have a 15 gal with a 1-1.5gpm pump and (2) of the grey 130 deg nozzles. Flow is based on pressure, the pump has a chart and the nozzles do too. Just match them up. 20-30psi is a decent pressure. I'd say 35 psi is ideal, below 20psi and the nozzles dont have a nice spray. Flow rates are color coded.
While you drive around, you learn how many gallons per acre your sprayer does. With my setup, I am around 13-15 gallons per acre. I usally put 1.5 quarts of 41% glyphosate (roundup).
In NY it's hard to get numerous kinds of herbicdes. Triclopyr is one of them. 3 Big common you can use.
Roundup / gly - kills many thing, stunts others, like clover and sedges. Good overall one for food plotters. Only kills leaves, turns harmelss once it hits the soil
2,4D - Kills many kinds of broadleaf weeds while not significantly harming grasses and other kinds of crops. Used in lawns and corn often. Careful, it hurts trees through ground contact.
Clethodim - Kills grasses while not harming broadleaf crops, vines, and tree plantings.
Colder water will have a little slower flow rate. However, adding chemicals reduces the flow rate a bit too. So, what cold water does now will be pretty representative of what warmer water with chemicals might flow. ATV's have trouble going slow speeds and limited what you can carry too. 20-25 gallons an acre is nice, but knowing what you actually can consistenly spray is more important than an ideal #. MAde many nice dead spots with my ATV sprayer. Spraying it by hand, the application rate is all over the place. Hard to make a consistent burn.
Also, herbicides have residual effects. 2,4 D and clethodim can damage seedlings you plant for 2-3 weeks. Glyphosate doesn't have an issue really. MAny of us spray and plant the same week, day, or even spread seed while you spray. Just make sure the seed isn't getting sprayed while you spread it. broadcast spreader up front, sprayer out back.