how to figure state income tax owed if unknown when completing federal tax form

Hello. This is the first year I am retired. I didn't have state tax deducted from our SS payments, pension payments or withdrawals. Now I am doing my federal taxes and I need to deduct state income tax which is an unknown. I could switch over and do my state taxes to get the amount but don't I need to complete my federal tax form to use the information on the subsequent state tax form. What is the workaround for this ??  How can I do one without the other ?

Thank you.