> if I lay it flat and then put the tape on, how will it flex in both directions. <

Not dumb, it is a good question. The key element is to have a small gap between the the two surfaces when they are joined. If there is no gap the control surface will up, but not down. This applies to both tape and to film (covering) hinges. I set this up on a trial fit with balsa shims for the gap and masking tape pieces simulating the hinge. This lets you check if the gap allows enough throw for the control surface.