For surface mount applications, your posts should be 3 INCHES LONGER than the height of your panel / gate. This will leave approximately 2 inches of clearance (space) between the bottom of the panel / gate.. and the ground. The 3rd inch is at the top of the post because the the top prepunched hole (line, corner, end, T and gate-punched posts) started approximately 1 inch down from the top of the post.
For stair-stepping applications, order longer, blank posts, based upon the rise/fall of your hill or slope. Blank posts can easily be trimmed on-site after installation. (punched posts cannot).