I usually just throw this coffee cake together and monkey around with the consistency until it is just right. Sometimes I melt a little cream cheese and put it in and the sugar can be adjusted up or down depending on how sour your starter tastes. I find frozen or dried fruit work a little better but I have used fresh peaches and fresh rhubarb before.
Stir all the cake ingredients together until well mixed. Add 2 cups of fruit if you so desire. I've used blueberries, peaches, blackberries, raspberries, dried apriocots, dried cranberries, chopped dates and rhubarb. I would up the sugar for raspberry or rhubarb. I usually do a light icing as well for rhubarb.
Pour the cake into a well greased 9x13 pan and tap the pan on the counter to help break up any air bubbles
For the streusel, mix the dry ingredients together and then cut in the butter using a fork or a pastry blender and sprinkle over the top of the cake.
Bake at 375 for 45 minutes and check. It will likely take at least an hour and maybe as much as an hour and 15 minutes depending on your oven.