thanks heaps guys, believe it or not, I had actually worked out the method Tom mentioned in my sleep. woke up at about 3am thinking "now why didn't I think of that earlier?" lol. dont know if it's because I have used it before or if I did actually work it out in my sleep but weird either way. 
that said, Lefty your approach seems allot simpler. I might try both out myself not only to double check lefty's maths 
but to also refresh my memory/understanding :).