GO with 50g bark/750ml water 50-75g lye (no such thing as too much). As for shaking, SWIY could in theory shake it for 5 minutes, let is sit for 30, should be 2 layers, shake again, and end over end turning. This is of course assuming SWIY has added the naptha. Shaking before will increase surface area (chemistry 101) which will expose more bark to lye. Adding more water can be helpful when you need to, but there is no reason to add more just to do it.
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