well, you won't overtighten unless you tighten it too much.
the o-ring under the atty can interfere, as it's hard to discern between "contact snugging" and "o-ring snugging". I usually put the atty on without an o-ring, and take a look at where it lands (rotation/position). However, without the o-ring, I can tell when I've made contact; so I don't snug it down as hard as I possibly can . .. . just contact, and then maybe a quarter turn. Then, when I put the o-ring under it, if it's not getting nearly as far "around the clock face" as without the ring, I know it's the ring giving me the resistance and not the metal-to-metal. (with the Nuppin', this is also . .. the dry fit . .. when I decide where my coils are going to lay out on the atty, and whether I'm winding coils with a right or left handed wind.)
Also, lately, if there's a call to make between a fatter or thinner o-ring, I'll go with the thinner until leakage proves me wrong.