Sheesh Paul, I dont thing you have to worry about the math that hard. I just eyeballed it, a bit of good 'ol ozzie common sense and rip into it.
This shed is 3600 by 2400mm and solid as.
http://fredsastro.googlepages.com/observatory
The floor frame was made with 2 by 4s, (treated pine) and tougn and groove for the floor.