You use the grease pencil or Crayola to draw the center line. This is done for two purposes. 1, to make sure that the hinge is centered between the two surfaces and 2, to prevent the CA from wicking all the way through the entire hinge and harden. With the grease pencil, you get a hinge line that stays flexible. BUT, as far as the CA hinges go, they're crap for this size plane. Go ahead and use them if your're a novice pilot and plan for this plane to go in during the first couple of flights. Not a problem. But, if you plan on having this plane for a couple of years, then upgrade the hinges. My first Patty, almost a year old with 262 flights had the rudder and elevator surface CA hinges go within the first 100 flights. Then I just changed out the ailerons just because I knew they would go too. Guess what, several of them were split too when I did change them. Save the aggravation and just change them out NOW! If you guys read through the posts, I dissected my first Patty after the fatal crash, and posted pictures, to see what these hinges were really going into. Stay right at the provided slots because that's where they're reinforced on the wing side. There's no blocking or anything on the control surfaces themselves, you're just gluing into the leading edge of each surface. I had a picture or two of what's there for you guys to look at. Given the material that's there, even though I put in the 3/16 Robart Point Hinges, I would definitely go with the pin hinges the next go round. I took 1/2" square balsa, 1" long and cut 1/2" square holes and glued the pieces in place. So, everywhere I have a point hinge, it is totally encased by balsa the entire length of each hinge half. Otherwise, only about 1/4 of the hinge would have been glued onto the balsa.

Experience tells me NOT to use the CA Hinges! BUT, it's your plane.