Since it wat rats, I think my DH used about a teaspoon, probably less for a mouse. Its funny, we also have possums running the fence as well. One time we heard the trap snap and ran and flipped on the back light to watch a possom flip the trap off his paw. My dogs love to ram the fence and bounce the critters (possoms or rats) off the fence into the yard. I think the critters have learned to hang on to the fence better if the dogs are around.
Good luck, I hope you catch the little bugger before he/she brings in friends or babies!!!!