For trees at least, the area to use for getting in/hr the area within the dripline of the tree. So the radius of the circle is the distance from the trunk to the furthers branch tip. Roots will grow at least as far as your dripline. Overtime, of course, this area increases as the tree grows. So you’re right that you can’t water new trees like the old.
The conversion to in/hr and other details of settings up drip (maybe not bubblers) can be found on a spreadsheet at Drip Emitter Calculator for Precipitation Rate & Area
People have used Spray instead and set the appropriate nozzle inches/hr (precipitation rate) to get soak. Bubblers can output a lot of water real fast, so soak might be a good idea if your soil can not accommodate its output over the time of watering, Now if you have water basins around your trees, then so long as they don’t overflow, you won’t need soak.
New tree roots aren’t too deep but you still want to water a bit deeper to encourage deep root growth. So you probably don’t need to water to the full 24”, maybe start at 12” or some depth beyond the pot depth the trees came in. You can increase the depth over time if you feel the tree would be able to reach deeper water. For my orange trees, I’ve set it to 12” permanently since they can get most of the water from the upper soil where the bulk of the roots are (you can do research on root depth, always fun!). Also as mentioned above, the dripline for new trees will not be large so the “area” you water will be less and that affects your precipitation rate (nozzle in/hr) quite a bit. Each year you can measure the new drip line area and adjust your in/hr until the tree is mature or as big as you like (and pruned to keep size/shape).
Most of the time the % number you’re looking at for soil moisture is the soil moisture balance. If depletion is set to 50%, the % balance only looks at the amount that is allowed to deplete. So… its 12% balance of that 50% allowed to deplete.