Converting a recipe to sourdough only
I found this interesting recipe on TFL, posted by David, and want to convert it to using sourdough starter only instead of the partial baker's yeast. Here it is: http://www.thefreshloaf.com/node/22023/horst-bandel039s-black-pumpernickel 
How much should I increase the starter amount to replace 4.6 grams of instant yeast?
Should I let the dough bulk ferment overnight instead of 30 minutes?
Should I encrease the final proof to a couple of hours or even more?
Any other suggestions to changes in process?
On the other hand, I will start out by doing this recipe as it is but I'd like to convert it. Beint a novice SD baker, I'm not altogether sure of how to make a conversion like that. I do like those Nordic rye breads and am thrilled with this one I frequently bake: