Alkalinity of 57 is pretty low. Without knowing much else about your water I don't think I'd bother trying to reduce the alkalinity.
I brew all styles with water that has alkalinity of 114. I treat it with lactic acid or use acidulated malt to adjust pH for the mash and I use gypsum as needed for raising sulfates in IPAs. Other than that I don't need much even per brunwater.
I've used straight distilled and I've used 50/50 distilled/well water and honestly seen no difference even in very light finished lagers.
Your beer, your process but I'd keep it simple.
EDIT: I use BS3 and love it but I'm not a fan of Brad's water calculation methods. They always come up differently from Brunwater and especially on acid requirements. I follow Brunwater's advice on water treatment. Just a heads up...YMMV