Put in a gallon of water and spray cement. Measure the wet area. Find out how many sqft a gallon covers with your setup. Mix accordingly.
Suppose you found that you covered 2000 sqft per gallon. Then you would need one ounce per gallon. If you used a four gallon backpack your would need 4 ounces. That would be enough to treat 8000 of your 10,000 sqft.
Now remember that treating bushes and trees requires almost double gallons per 1000 sqft because of all the vertical surface you need to cover.
Make sure the poison ivy is not mature or dormant, but in a stage of rapid growth. And true, it is not on the label.