Scaling down ingredient quantities using baker's percentages

Until now, I've used recipes from the good folk who post on this and other forums, or from books like Dan Lepard's The Handmade Loaf catering specifically for the home baker. I've always either followed the ingredient weight as per the recipe, or if it looks like too much bread for my purposes, I simply halve the quantities.

I recently bought Hamelman's Bread and would like to try his SD semolina bread (p. 171), but I have hit a mental brick wall trying to scale down his quantities. Those who have the book will know that he gives 3 sets of ingredient weights per recipe: U.S. imperial (lbs), Metric (kg) and Home (oz), as well as baker's %.

His 'Home' ingredient quantities produce "2 large loaves" - I probably only want to bake the weight equivalent of 1.5 loaves or so (don't ask!) - and are in ounces. I don't like using anything other than gms, and I'm thinking baker's % is most easily used to calculate ingredient weights for the total weight of bread required. I have an Excel sheet that theoretically makes these sorts of calculations easy, but even with that, I am currently struggling.

My problem is, Hamelman doesn't give weight or baker's % for the levain component in his "Overall Formula", although he does give the weight and baker's % of the levain components separately.  My brain is further addled by the fact that he makes 0.3kg more levain than the recipe needs (to keep for use in the next bread), so that presumably has to be subtracted from the levain total weight if I want to make only enough for the recipe. All this caluclating is starting to look like a pain, but I want to push through and get on top of this scaling down stuff because I am sure that Hamelman's recipes are as good as the raves suggest. And I've got the book now, so want to use it!

Would be most appreciative if a more experienced bread wiseperson could lead by example here, and calculate Hamelman's Semolina Bread ingredient weights and the levain weight required to make, say, 850gm of dough . Once I've seen it done once, I'll be fine.

Hope the above makes sense. Reading it to myself prior to posting, I have some doubts as to my intelligibility...

Cheers
Ross