Hydration whithout whole wheat

I would guess just a little less than 70%.  Why don't you just mix as the directions call, but hold back some of the flour and gradually add it in until the dough feels right.

Ford

This is just a cursory look at what you're talking about:

A (bread flour)

B (water)

C (sourdough starter) = D (sourdough flour) + E (sourdough water)

If using a 100% hydration sourdough starter the math is easier as D and E are equal, so just divide the weight of your starter by 2 and that will be the same as D and E.

You want a 70% hydration dough so your math will look like this:

0.70  =  (B  +  E)  /  (A  +  D)

For example,     A: 475g bread flour

                                B: 325g water

                                C: 50g of 100% hydration sourdough starter  (D: 25g flour + E: 25g water)

                                (B+E)/(A+D) = (325+25)/(475+25) = 0.70

                                I of course chose numbers that made the math look pretty, but you get the idea.

There are other posters here who have done much more elegant analyses of this concept.  David Snyder has some that are particularly useful...

Baker's Math: A tutorial

Converting starter hydrations: A Tutorial.  Or through thick and thin and vice versa

You can also find lots of stuff like this here with the search function.  Hope this helps!

     --Mike