Generally as little gap as practical.
The technique for the "Monokote Hinge" serves as a good guide for any film based hinge.
This assumes the surface will be "top hinged".
1) Lay a piece of card stock on the fixed surface to act as a spacer. Typical cereal box material is good when using Monkote.(thicker material if the tape is thicker than Monokote... maybe double card stock for "Blenderm" Triple for duct tape.)
2) Lay the control surface inverted on the spacer material. (you will be looking at the bottom of the control surface (assuming aileron or elevator)
3) Line up the edges that will become the hinge line.
4) Apply the lower surface hinging material.
5) open up the result and you will see the gap between the fixed and moving surfaces (sealed by hinge material) Bring it on to full deflection opposed to the position used to apply the first hinge material.
6) Apply the upper surface hinging material.
The hinge is complete.
This works wonderfully for balsa structure with a SHARP 45 on the front of the control surface and sharp 90 deg corners on the TE of the fixed surface.
If you have rounded edges then you need more gap.
If you are combining pinned hinges with tape to seal the hinge line you need more gap.
Learning how much gap in different situations (foamie, centered < LE on the control surface, large radius to the < and backing up pinned type hinges with gap sealing tape can mean a BIG gap)
Always apply the tape/hinging material with the control at max possible deflection and get the tape as far into the hinge line as possible. Tape not fully in the gap will cause binding.