Hi,
Well it really can vary on the type of yacht, some do have a lean back [not much] depending on the runing rigging, some have a bend forward in the top portion called mast bend, generally older yachts tend to be perdedicular to the deck, but also the cut of your sails can make a huge difference.
Perahps some old photos, plans might help.
Have a chat with Frank at Nylet, he will certainly help.
Rich