you gotta wait till they make first contact if you want to deal with the legality of your action...saps are illegal offensive weapons as you know,,,while your forehead is as hard/strong as it gets while his/her nose is an obvious weak spot...
imagine this,,just walking along,,minding your own business,,wearing a hoodie, face obstructed, looking DOWN and suddenly somebody blocks your path and touches you...unsolicited touching is assault, once that bar has been reached, you are well on your way to proving self defense as you were blindsided by an assault and only reacted instinctively...
"gee,,,i didnt mean to smash her face into pulp, but she surprised me with her assault and i was just trying to escape when we "bumped" heads"...
your forehead is one of the best weapons you carry..it may sting for you, but its devastating for the recipient...