The inevitable next beginner question

Now that my wild yeast starter is perking along (How do you get a starter going?  Post a whine on this website that it's not going and--whammo!  It takes off!), I'm dreaming of actually USING the starter soon. . .

One of the reasons I wanted sourdough was to add to the multigrain loaf I've been baking (recipe here:  http://www.kingarthurflour.com/recipes/multi-grain-loaf-recipe) .  The bread is delicious, but stales quickly.  My thought is that the addition of some sourdough will improve the longevity fo the dough.  I do not intend to use the sourdough as the leavening or sour flavor.

So how much starter should I put in and how much of the AP flour and water should I remove from the original recipe?  Am I correct that 100% hydration starter has an equal weight of flour and water, therefore if I add 60 grams of starter I am replacing 30 grams of flour and 30 grams of water in the original recipe? 

How do I determine how much starter to use when I am not seeking to leaven with the starter or affect the flavor?   Should I start out with a small amount and gradually increase that until I hit on the right combination?  Or is there a general rule of thumb proportionally?

That recipe calls for KA "dough improver" to lighten the structure a bit.  I don't have the KA product, but I have been substituting 1 TBSP of vital wheat gluten and 1 tsp of vinegar with good results.  If I'm adding sourdough starter, will it take the place of either or both of these ingredients, or should I still add one or both? 

I'm not going to try to add starter to my multigrain loaf this weekend, as th starter is still very young.  But I am going to try sourdough english muffins.  My husband has to go out of town to help his mother move to assisted living, so I am cooking my own mother's day breakfast for my kids--after all, they MADE me a mother.  And I'll get to try out my mother (dough) for the first time!  I am looking forward to homemade english muffins slathered with butter and jam!