This used to be a topic on the rafter thrust calculator only. I have redone this as the portal link to his assortment of online calculators.
They may be accessed from his index page HERE (http://theownerbuiltcabin.com/calculators/TT/index.htm).
I have found several to be valuable tools when designing or modifying a design.
Some of the calculators require an inpit for roof pitch. Pitch is usually expressed something like 12/12, 6/12. etc. When asked to enter roof pitch the description field will be something like... Pitch of Rafter (e.g. 6/12) Enter only the part ii Red like 6 in 6/12. (the numerator is one views 6/12 as a fration)
In the Rafter Tie Connection Thrust Calculator results field "Tension in midjoint of ceiling joists (lbs)" the calculated result can be interpreted as the amount of force that would be pushing horizontally outwards on the wall at each rafter tail, if the rafter tail was not restrained by a rafter tie. at the position where the rafter, rafter tie and wall top plate meet.
My website above was hacked by a squatter. I've begun putting the calcs on a new website;
http://theownerbuiltcabin.com/