marty, that just looks like a lip gripper...not a scale? is this true? that just looks like something that will hold onto the fish, not weigh it.
anyway, i got a 50lb berkley digital scale ($20 at walmart). totally worth it. accurate within an ounce. i've tested it on a lot of things (fitness dumbells, groceries, 5lb bag of sugar, etc...) and its always within an ounce.
one thing that i do is sharpen the hook and put it right behind the bone on the lower jaw, just like marty said. this seems like the most secure way to weigh a fish. i've tried hooking them under the gill plate, and i've had some nice fish flop around and fall off the hook doing it this way. also, im not sure how good is it to support the weight of the fish on the gill plate. im sure the bone can handle that much weight much better than a gill plate.
to summarize:
-$20, 50lb berkley digital scale at walmart = good
-sharpen hook on scale and insert just under lower jaw bone