please just a few more details …
is this a tension fitting ? a discrete fastener or one of many ?
how do you hand calc ? did you include preload ? prying ?? (this should tell you something of what you need to model)
how have you modelled the joint and fastener ?
have you tried YT ? (many times videos have been prepared for this type of question … how do I XYZ with ABC code ?)
oh, and why ? (are you doing FEA on a nut ? I'm picturing a typical nut and bolt, but your "nut" could be on a 6" dia stud … that sounds a lot dirtier than I meant it to be)
another day in paradise, or is paradise one day closer ?