I like Bud's approach.. I had a concern about the pencil method ( I have a heavy writing hand)... So, I used some 'blue' painters tape to mark the underside of a level.. one piece each end.. that will provide a 'level' mark.. now, start the tape on one of the tape pieces and stretch across to the other... semi-instant straight line..
you can now mark the blue tape where you want your letters to align..
Apply as directed.
Oh, I measured center based on the edges of the front upper clearance light housings.
Peace and Blessings..