There is no correct position, I'll generally just pick somewhere based on the look. As long as it's low enough to miss your panel and do whatever kind of lid attachment you want it's fine.
For me, it mostly depends on how the lid is going to be attached, the size of the box and if there is any specific bit of grain I want to keep/split.
Little boxes I generally seem to go for about a third, but less than that on bigger boxes.
