All structural design is done by one of two methods: 1) Ultimate Design, or 2) Working Stress Design.
1) As you might guess, ultimate design is based on “failure” of the material. When that method is used, a safety factor is then used based on the members importance. (Usually 20% to 100%...based on the designers experience and code minimums.)
2) Working stress design has already accounted for these safety factors.
The values you list are a bit confusing, because for compression perpendicular to grain for “working stress design” is 385 psi for Douglas Fir-Larch and pine and Spruce are much less, (they list 425) but compression parallel to grain for “working stress” is 1500 psi maximum.
This is all based on many factors, a) species, b) grade, c) use (repetitive or single use), d) size of member, etc. I suspect they have given you a safe number to use.
So, to answer your question: the maximum load for your 2x6 jack stud (I call them trimmers) is: 1320 x 1.5” x 5.5” = 10,890 lbs.
However, it sits on a plate and the header sits on it in perpendicular compression, so the maximum load it can support without crushing the plate is: 425 x 1.5” x 5.5” = 3,506 lbs. (neglecting the nailing into the king stud.) Note: I used their value of 425. You can substitute 385 if necessary.
