How do I change who can see my posts on Facebook?

[pri✍]

You already in the right track, set the audience setting to Custom; with that you get the option to tell FB whom you want to not see the posting.

Now, if your concern is you don't want your family to see that, then tell that to FB.
How?
Go to your Newsfeed, click FRIENDS category, then click Family list. Add your family in that list.

Now in Custom setting, you just put Family list in the 'not seen/share by/to'.

Okay? Good luck.