Ed Levin's articles on Joinery Engineering in*Timber Framing* issues 38 & 39 explains how this can be calculated algebraically or graphically thru vector analysis (free body diagram); those articles are also in the Guild's Joinery & Design Workbook Vol. 1. I bet you could find some tutorials online on an engineering website.

I don't have Steve's book with me so can't address his specific example, but you take that 1600 lb. thrust on each rafter and extrapolate it to each post based on the number of rafters per bay. Note that the actual moment and stress on the tie joint in a high posted cape is magnified by the length of the cantilever above the tie beam, and this determines the joint design and size of the post required. So the thrust is only part of the calculation.

Here's a quote from Ed's article:

"Whether you come at it by the algebraic or the graphical method, it's possible to generalize a principal about thrust in simple rafter roofs: The resultant force always acts at an angle to the level whose pitch is twice the roof pitch. So in a 12: 12 roof the vector sum of thrust and gravity load is pitched at 24:12, in a 6:12 roof the resultant acts at 12:12, etc. Hence roof thrust can be quantified as follows: For simple rafter roofs (no collars, struts, kingposts, etc. to muddy the waters), with roof slope S (in degrees), the thrust is equal to the roof load divided by twice the tangent of the slope, or Fx = Fy / 2 tan S."