When do you map an ambush?

In a 300pt 28mm tournament game ambushes are marked on a map, when is this done for the attacker and defender? For example -

The attacker can see some or all of the defenders army before he deploys, is this when the ambush is plotted or must it be done before any defending units are on table?

The defender may be able to hold some units back depending on the General's rating. Can these units be marked as in ambush once the attacker's first deployment is made and the defender can see what's on the table?

Cheers

I've always assumed that the ambush map was drawn after the terrain was placed and before deployment. So with ambushers already allocated, any retained bases would be extra. May be wrong though...

as Aurelius says