How do I properly substitute flour and water for sourdough starter?
Starters are typically maintained at 100% hydration.   That means equal parts water and flour.  So, in your case, mix equal parts water and flour.  Measure 1 cup of that, and add it to your mix.  Of course, this will mean that all of your starter is gone.  Alternately, feed your starter (equal parts water and flour) with more than you need, and let it sit on counter over night.  Then you you can save the left over to use again next time.