Cleft grafting is my most successful practice with mangoes, my veneers are getting better, but definitely suggest starting with clefts. Ideally your scion and graft target are exactly the same width, simple then to match the cambiums as long as your cuts are true. If your graft target is larger than your scion width, it's still pretty easy to split your target off-center, where it matches the width of your scion. If your scion has a greater diameter than your graft target, it's gets a little trickier. I usually go for a veneer in that case and hope for the best.