RoCo IV suggests (5.13, p 36) that you can apply level bonuses to Ambush. You divide your total level bonus by two and that gives you your bonus ranks (I'd round down, because it is a powerful option). I wouldn't use that option myself, as your 10th level Nightblade would have fifteen extra ranks in Ambush, as compared to a +3/lvl bonus to Silent Kill which would give the equivalent of 6 ranks at the same level. I suppose, given that level of equivalency you could divide your level bonus by five instead for a more balanced result.
A better way make those level bonuses count might be to go the RMFRP route, giving a standard 5.2.1.1. etc progression for Ambush, make the character roll for success using that bonus and then, if successful, apply the skill ranks (not the bonus) as per the standard rule. This would work for the other skills you mentioned, too.