correct 3 bay victorian conservatories are at 45 degrees - or more accurately at 135 degrees
there are lots of trade software around - such as
http://www.roofwright.co.uk
however I imagine lots of the relatively low tech home design packages available on the web will be sufficient for mosy laypersons requirements.
have not tried this - but it looks promising:
http://www.artifice.com/dw_classic.html
Do allow however for "thickness" of wall when calculating internal floor area