Let me try again. I don't really know of a rule of thumb for cooking fish or shrimp. I use the finger AND thumb method. There is discernable, identifiable softness. The instant a shrimp begins to firm up, it is done.
Cooking longer makes it tough and chewy.
The flesh should be moist and the texture should offer some resistance to the teeth, but cleave cleanly. Fish texture will depend upon the characteristics of the fish. Normally, only lean, firm fleshed fish are grilled. The color will go from semi-transluscent to opaque shortly before it begins to flake. Press the thickest part with your finger, if it does not feel soft, it is done.