Trying to use Mate is a shambolic experience - simply a waste of time and massive aggravation.

I have a shed and trying to 'design' custom shelving. (completed on paper/pen 20minutes). 12 2"x2" boards with 4 each at 3 different lengths. 5 hours of frustration of Solidworks.  All 2"x2" boards are at weird angles to each other.  No 2 surfaces will mate in any logical way.  Corner/surface, surface to impossible backside surface, surface offset/surface offset,  surface/ center of other surface.  I wish there was a way to tell SolidWorks - Work Bench surface (VISIBLE reference point) which I can build on.  "Normal to" will relate all  selected surfaces/edges to Work Bench surface.  Rotation of parts are based off of Work Bench surface.  As of now: try something, see results - 99% failure - try undoing that something - 99% failure- close SW - no save.  Start again.