IF MY BOARD IS LOCKED DOWN ON ONE END (FOR EXAMPLE, THROUGH USING THE SECOND SCREW HOLD ON THE SIDING CLIP), HOW MUCH SPACE DO I NEED TO LEAVE ON THE OTHER END TO ALLOW FOR EXPANSION?

This depends on the length of the board. An 8-foot board would need 1/8th of an inch, a 16-foot board would need ¼ of an inch, and a 32-foot span (two 16 foot boards in a row butted up where they meet) would need ½ an inch.