honestly, it doesnt make sooo much of a difference in SWIM's experience..
SWIM already did extractions with 1g bark:1g lye:15ml water, and it worked fine.. He also did 1g bark:0.7g lye: 15ml water, also worked fine. He already also did 1g bark:0.3g lye: 9ml water, also worked no prob.. All of them he got around 1% yield
less water makes separation of solvent slower, but doesnt affect the yield. SWIM has heard that with less water than 1:1:15, the solvent doesnt separate at all, it gets trapped in the thick mimosa acqueous layer, but it never happened to him.
Less lye, with powdered bark, also doesnt make so much of a difference, at most it increases the chances of emulsions, but SWIM stirs and doesnt shake, and the few times it did happen he either added a bit more lye or pure salt and it made the emulsions disappear
at least thats how it went for SWIM