Bud,
Suggest you re-measure the new material you will be receiving so you have the 3 inches each side for mounting and framing. By the 15 7/8 inches wide by 20 3/8 inches high you previously mentioned is the completed stitched area, your new material (and I rounded up to the next WHOLE NUMBER) should be at LEAST 22 inches WIDE x 27 inches HIGH.
If you don't see the arrows and want to start in the Upper Left corner of the material, then measure 3 inches IN from the upper Left EDGE of the material and make a light pencil mark; then measure 3 inches DOWN from the upper Left Edge of your fabric and make a light pencil mark. Where the vertical and horizontal pencil marks MEET is where you stitch your first "X' to begin the pattern in the upper Left corner.
Be careful NOT to cause any "ridge" lines when going from one page to the next. A few variable "overlap" cross stitches from one page to the other will help prevent the "ridges" from occurring.
remember: Ruffles potato chips aren't the only thing that can have "ridges"....:)