I have done this by getting a sheet of paper and a marker. Draw a box on it 23.5 cm by 10 cm or use inches if you have a larger sheet. Mark the center point and pin it on the wall an point the camera on it. Zoom in to get the vertical lines on th edge of frame. Then go into the camera menus and adjust the TV safe area until the horizontal lines hit the lines on the chart (I think it is about 80%). Shoot some of the chart so you can put it in your NLE and use the lines for a mask there too.
Nigel Ellis