FWSY 80% Biga, 20% WW

On my quest to get better at batards, I baked FWSY's white 80% biga recipe and subbed 20% of the flour with WW. 

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I'm finally getting some good scoring. I think my razor blade is too dull, I have to go back a second time through to get a decent score. I also think part of my issue with batards falling flat as soon as I get them in the oven has been to do with my proofing baskets. I think my batard bannetons are actually pretty small? (https://www.amazon.com/gp/product/B006J7KBPW/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1) When I've tried a whole 500g dough (half of a FWSY recipe), it almost seems like it's too much dough for the basket. Anyway, I've quartered the recipe so this is half the amount of called dough per loaf. 

Baked at 460 (per David's suggest when using a stone instead of DO with FWSY) for 15 minutes covered and 20 uncovered.