That looks like about a 7mm gap. So if you wanted a 2mm action, you would have to add a 5mm shim under the neck, if they were parallel shims.

If you used the suggestion of a tapered shim, it would have to be around 1mm at the thick end. That is assuming the length of the neck/body contact surface is 50mm. You could test it by putting a 1mm shim - like a match stick shape - right up in the neck pocket to simulate the thick end of the wedge, and see how the action looks then. Or you could cut 5mm wide strips of business card and put them in the corner of the neck pocket.