Essentially you are separating the layer from the tree beneath it, so you need to make sure the area you want to layer is putting out good growth, and wait for the leaves to harden off. Typically layering is done right after new growth has hardened. It shouldn't really matter when the parent tree was repotted...I think... Anyone else?
The thing is, I want to and believe it needs a re-pot again next spring (in the same training pot) and trim the roots then as well. Wouldn't it be best to spread out the stress instead of doing it all at once? Say late July, early August start the layer and ideally get it cut by September?
The key is for the tree to be healthy and growing strong. You might want to consider putting it into a larger pot for this process. You would not necessarilly need to prune the roots. Also, let the top grow free. Don't be trying to stlye the tree while you are in the air layering process. Lots of fertilizer.