Converting starter to barm sponge, possible? How?

Hi, I am in need of help again. I am struggling to work out whether I can:

1. Convert my starter to a barm sponge

2. The calculations to do so

I have starter which I believe is kept at 100%.  Equal weights of flour and water? I want to try and bake Peter Reinhart's San Fransico sourdough, (from Crust and Crumb pg 74),which calls for a barm sponge starter. The final days refreshment for the barm is 4 cups (18oz) flour and 3 cups of water. Which he says works out to 3 parts flour to 4 parts water by weight. I think this means that I need to refresh my starter next time with three quarters of an ounce of flour to every 1oz of water, I think I can manage to build my starter up to the required 18 oz that way, but my head won't get around the maths of including my original starter at 100%. Could I just weigh out some of my original starter and add 25% of its weight in water and give it a mix? Would this give the correct flour to water ratio? Would it matter if I need to refresh a couple of times to get the 18 oz if my original starter was 100% and I didn't adjust it all, just refreshed at the correct %?

I am not sure that I have explained that very well! Sorry. My brain finds numbers much more difficult than just about anything else!

Thank you