I would recommend waiting till its a bit bigger. That way you can make two or more grafts by chopping it up, you'll double your chance of success, or half your chance of failure. It is best if the scion is more than a couple of cubic cms for that kind of permanent stock grafting. Just makes it much easier to handle and more likely to survive.
I would also say that you should 'pencil' the end of the stock and then cut it where it is the same width as the scion. Otherwise when the cuts dry out, they start to shrink and move and they can push apart the graft union before it forms properly. If you pencil it, it will mean that the attachments elestics are pulling more downwards as oppose to out from the center.
If it is a crest it is special and you should do a bunch of practice grafts first to get the hang of it. Even the best grafter started out with a high rate of attrition.