Residential Structural Engineer Required

I am in need of a structural engineer that can calculate a load limit for a piece of fiberglass reinforced HDPE post. Looking for a quick analysis based on the attached specifications from the manufacturer. This product is considered as structural lumber and is supposedly used in a myriad of applications as a load bearing component. Would like to determine the max safe load on a nominal 6x6 post that is directly bearing on the center of a 20” square pice of sandstone that is 3” thick. The sandstone would rest on an approximately 12” base of compacted gravel that is itself resting on undisturbed clay. For this analysis, we would use a bearing capacity of the clay earth support to be 1500 lbs per sq ft. Not concerned with frost line and ground movement regarding the clay base. Just looking for a simple Excel chart showing load vs post length in 6” increments, and possibly an ambient temperature correction factor in 10 degree F increments from -10 to 140 degrees Fahrenheit. Assume the top 12” of the post is inserted into a steel cap, thusly controlling expansion of the uppermost portion of the post, and the weight resting directly on the cap. So for a 2’ post, only 1’ would be exposed below the cap, and its 6”x 6” end would be resting on the sandstone.