1 flour + 1 B fresh water -> 2x dough. Same ratio as crafting, just slightly more convenient when crafting in bulk (although may still be preferred if you want to just craft 1-2 instances).

Instant?

Never mind, this wouldn't work as many flours are used for alcohol. If the issue is to be addressed it needs a different approach.

Maybe make it so 1 bucket can "hydrate" up to 8 flour, like, you can craft a bucket with up to 8 flour (in a crafting bench)