the "calcs" will take lots of things into account, including over engineering the design so that it will take more load that you could posibly apply to the structure

i think omndeo man does this sort of thing for a living, drop him a pm to see if he can shed more light on the matter, but it's a little soemthing like this:

wieght of the roof
wieght of the load on the roof
roof materials etc
then you've got the loading for the top
then you can work out how big a set of joists to use to support all that
then you can figure out how much studding you'd need to carry all this weight
then ultimatly down to the depth of the foundations needed to carry all of that wieght and so on, so more than just a few taps on a calculator

also make sure that the drawings you get match up to your house dimensions, if they are wrong then you've wasted your money