Notice how he is measuring the board - not with a measuring tape, but with the frame itself.
To do:
1. place center brace at center mark on the frame. Have one end touching the floor and bring it up to the edge of the other end and MARK with a pencil. REMEMBER: it is better to cut too LONG rather than too short!
2. Cut your board with the miter saw at the mark - set the saw at the center setting and just cut straight down - NO ANGLES are needed!
3. Bring cut board over to the frame and see if it fits. Your goal is to have the center of the brace hit the center line of the frame (see bottom photo). This fit should be snug, but not too snug. You may need to cut the board a few times to get it right - no worries, just be patient.
