Frye, please do not be offended, I mention this only for your own good.
It feels like you are over thinking this. A straight and true extension of a wall.. does not need to be a mathematical equation.. your lazer will only give you a point to point referance.
get some wood L blocks for masony.. 3 sets for.. low..middle... high..
and a ball of string. Set a guide post past the new corner...and plumb it.. block out the string... and make you plan..
With care you will be able to make your new wall just as imperfect as the old wall...
and keep it straight and good to the eye.