It looks like your amount is right, but you should expand the diameter every year, as much as you want for diameter, always beyond the drip line to keep improving soil. Eventually the tree will provide its own shade and only the outer edge will ned something. You might try a different type mulch from time to time. Maybe use leaves, then straw, then horse bedding, rakings from under bamboo, coffee grounds, compost. By changing the source of mulch you may be adding diversity.