If my ex-wife quit work, do I pay child support on what she was earning and her potential earnings based upon her degree as a nurse?
There's no hard and fast answer: it depends generally on "why" she quit. To oversimply, but convey the general idea: if she quit voluntarily because she did not want to work or was counting on support from you to let her live without working, the courts generally would not let her profit by her decision and would set support levels based on what she could earn.
But if she quit for good reasons not under her control--such as the child or children needed a full-time parent to care for physical or emotion problems, or because she can't find some other caregiver to help out (e.g. no family nearby; can't afford a full-time nanny or day based on what she was earning and it actually makes economic sense to quit her job; etc.), that would not count against her.
Also, courts are generally more generous about child support than spousal support, and tend to set support levels at the level or amount they feel the child(ren) need. So she will be more likely to "get the benefit of the doubt" on child support than she would on spousal support.
If you feel you are paying too much under the facts of this case, you can challenge the amount in court. Doing so can be tricky; you are advised to retain an attorney to help you.