alan,
Geometrically speaking, if you have a symmetrical rectangle or square the positions of the marking will not change with the opposite sides. Or in other words if you have a rectangle or square and you flip the piece and mark then cut you will have the layout correct and it will fit. so if you have all the thru tenons 1" in from the corners and each side; top or bottom, side to side, they will all be the same. that being said, you must first make sure everything is square, and if you're like me, they're often a little off.