Hi Ahsa, That’s such a great question! I’ll be honest, I just usually line up the mini molds I want to use, then have a bunch of other small molds on standby to fill as needed with leftover batter.
I feel like there’s probably some math we could do to be more precise – perhaps try filling each mold with water and seeing what the total volume is?
So if your combined mini molds hold 14 ounces of water total, then aim for your soap batch to weigh 14 ounces total. (Lye + water + oils = total weight)
It still won’t be precise since water will weigh differently than soap batter, but it might get you somewhat closer to making the right amount of soap.
(This is my best guess off the top of my head – someone smarter than me might come in and have a different/better idea.) ?