PS -- One of the harder aspects of shield budding mangoes is getting the bud eyes to spring, depending on your timing. If you're bud is still green after a month in warm weather (perhaps 6 weeks in cooler weather), then your graft is good and you just need to figure out a way to get the bud to spring.
Here in FL, shield budding from Oct to Dec was best for me, as bark was still slipping during this period, and the spring push is so strong, that nearly all bud eyes would begin growing on their own in April. If you're on your game and want to crank out hundreds of grafts, you plant the seeds in June. Then in Oct they are ready for budding. Then in spring you have little grafted 1 gallon trees growing. (The timing would probably be different for socal.)
If bud eyes don't spring on their own, you need to head back the rootstock, leaving only a couple of leaves above the grafted bud. You can also head back the rootstock at time of budding -- just keep an eye on the shoots that try to emerge from the rootstock (pick those off).
But if you are not grafting dozens / hundreds of trees, easiest is to just cleft or side veneer. A great way to get instant grafted trees (with high rate of success) is to graft onto seedlings when they are literally only 3 weeks sprouted (with foliage still reddish), using a cleft graft. Rate of take is high, and there is still plenty of energy in the seed to push scion growth (you don't even need to leave foliage on the rootstock, though it can be helpful to leave a couple of half-leaves on the scion and bag the whole thing).