Hi Tina, you can generally substitute 1/4 cup water (57g) and a scant 1/2 cup (57g) of flour for 1/2 cup of sourdough discard. However, since cookie recipes don't normally contain water, and the discard starter is likely thinner in consistency than the combination of water/flour suggested, I would begin by substituting an egg and 50g of flour and see if that yields the proper consistency for your cookie dough. 

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