It makes sense that the overnight FC wouldn't change much with so little of the algae's surface area left exposed to chlorine.
I think Borate is a good choice - maybe a little higher. The pH might be getting a tad high (especially with 400 CH). Muriatic Acid is a good way to bring it back down - Be sure to read and understand:Using Muriatic Acid to Safely Lower Your Pool's pH.