You have asked an interesting question. Of course these forces change all
the time as the wind, sea state, heading and sail set change. If you knew
these numbers exactly, what would you do with the data? A prudent designer
would multiply the forces by a safety factor (like 10 or more) and then
develop the stresses. For practical purposes, assuming you are interested
in rigging a real boat, take a look at similar successful boats in your area
and plagiarize.