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Tight enough so it doesnt come loose? I dont think they need much. Just give the nut a little oomph crank and it should be fine.
I use a 1/4 inch drive screwdriver like extension with a deep socket and tighten them this way so as not to overtighten them. Remember they have an O-ring that does the sealing and you don't want to distort it too much.
I put a light film of oil on the o-ring to help seal, then I just hand tighten gently. Never had an issue. I guess the wrist just clicks when it is right.