Well, i have limited experience, but there are a couple of things i can say, based on what i have seen (and i hope they may be useful for those starting, like me).
Firstly, 2 graft (of the 3 i have done) seem to have taken (today i controlled even the others i made, i spoke about them earlier).
The 2 which did take were about to push when i collected the scions (you know, when you realize that the terminal bud is about to start developing, but isn't really developing so fast YET). Both of them, and even the rootstock were in this phase. I guess that if you manage to keep the scion alive for very long (like in your case) you can graft even on a rootstock wich isn't pushing right then, but seems a bit risky to me (honestly, is hard to me understand when a mango will push new grow, even if i noticed that, when well hydrated and healty, they put out basically a flush every 3-5 weeks, so i wouldn't graft on something i don't see growing).
The one wich i grafted and didn't take (it whitered very soon apparently) was a scion coming from a flush wich was just hardened, and even if it was pushing when i have taken it, it dehydrated quickly once i put it in place. So i guess that best material comes from scions wich are about 2-3 flushes old, well hardened, woody but with green skin.
Well, that said, another thing i found useful is having done everything with scions and rootstock of the same size. Maybe the experts are able to match even scions and rootstock of different sizes; i'm not.
That said, having the rootstock fo the perfect size, at the perfect stage of development when you want to graft is a bit tricky, expecially if you haven't much space. I guess that it is the rootstock wich actually decides when it wants to be grafted.
