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you can also use painters tape the or a piece of chalk or white colored pencil.
I use the White Pencils. If you keep them sharp then you are all set. Liquid paper is a pain the way you are doing it. The pencils do not cost you and arm and a leg.
Let me know how it works for you.
491 Kennard Road
Manchester, New Hampshire
i would use a strait edge and a razor blade to mark the center