I'm all clear! (and a little shamed)
Thanks so much for the excellent replies.
I was subtracting 180 from the solution rather than subtracting the solution from 180
essentially just trying to use the reciprocal and my head was a little fuzzy from 3am beer and tiredness.
Plus i'm a bit of a trigonomical dunce to begin with
You guys, on the other hand, rock. What a fanstastic forum.