We apply in a 15" grid pattern from the trunk or stem to beyond the dripline a bit at 200 PSI with a 3/8" hose using a GreenGarde root feeder. "Deep" injecting is not a good practice because roots that take in fertilizer and water are close to the surface. It's best to treat from just below the surface to about 4" deep in mulched or soil areas. If you are fertilizing tree roots covered by grass, insert the probe about 6"-8" deep and inject. Any deeper is probably not going to benefit the tree or shrub and could lead to leaching.
Fertilizing trees is realy based on the square footage of the tree. You have to guesstimate the height and width and then go from there (unless you want to pace out from the tree and do a little more math). Use a 3' x 3' pattern 3/4 of the way to the drip line no further and go down 6-8". Your typical tree in the 8" caliper should take about 10-15 gal, using 1qt per hole. You have to calibrate your discharge for that. I use a soda bottle and count to 4 or 5 and adjust as needed. If you use Arbor-green, mix it at a rate of 40lbs per 200 gallons and you should be all set. If you want more specific rates I will bring home my secrete book and give some numbers. Figure on 1 hour per 100 gallons plus materials to get your price. It is easier to sell it that way and give the estimate for the property to include foundation plantings ect.