You need to establish some depth indicators on your bucket at teeth down and back down positions as well as several indicators on the stick. Until you entrench these numbers you might go ahead and spray some paint on the bucket and dipper at depths you use. Work hard at learning to make the finish cut with the back of the bucket flat in the bottom and the teeth just barely cutting out teeth marks, etc. It will require operating boom, dipper, and curl all at the same time.
Know where 18" is. It is usually safe to dig to that depth. Get scared at 18 and stay scared well after 24 as most things are buried at that depth, at least around here with a frost depth of 18. If there are utilities, back off the throttle and keep the teeth open as you work the bottom of the ditch. Curling those teeth sure can bring up some pretty colored wire and pipe. DAMHIK.