A. Watering amounts are usually related more to the size of the canopy or total height than the trunk. Trunk size is sometimes used more for estimating how much fertilizer to apply.
That tree size would require around 10 to 15 gallons for each watering. If you are using drip emitters, you should be using four (4) emitters in a square pattern 12 to 18 inches from the trunk. So running 3 gallon per hour emitters for one hour would equal 12 gallons.
Just to remind readers we follow the PET curve developed for here and is below.