For iowa state reporting you report federal income tax refund received in 2018. Last year I was married filing jointly, this year I am divorced. How do I calculate it?

Prorate it to your taxes withheld last year.  For example, if you had $3000 of tax withheld, and your ex had $2000 withheld, your tax refund last year is 60% based on your withholdings ($3000 of your tax withheld/$5000 tax withheld overall).  Multiply that percentage by the amount of Federal Tax refund.  If your tax refund was greater than the amount of tax withheld, then you only include your taxes withheld, and you are already pro-rated.  (The amount of refund received that you must include on the Iowa return cannot be greater than the amount of Federal Tax you had withheld from your check last year.)