For halfsies I use a combination square, set to approximate half, mark, flip top the other side and mark, and dead center is the midpoint between the marks. Then hold your pencil on the exact midpoint, reset the square to lay against the pencil and zip it down the length. If you’re really anal, do that twice – once from each side – to account for possible width variations. Of all my tools, my little 6" combination square gets the most use when woodworking.
Losing fingers since 1969