A precise way to make hinge slots exactly centered is to use a Dremel arbor and a "cut-off" wheel in your drill press. Adjust the wheel to be off the table by 1/2 the thickness of your stab, elevator, flap, etc.
Once a slot is cut, turn the piece over and run it again. The slot will then be exactly centered. If you have adjusted the Dremel wheel accurately, the slot will be equal or slightly thicker than the thickness of the wheel.
I always try the adjustment on a scrap piece to get the table set as close as I can to the center of the balsa piece.
Any setting error is cancelled by running it through twice, once with the piece upside down.
Of course, wing trailing edges must be slotted before the wing is assembled.