How low can I go on coconut? Should I use the smallest amount I can get when I plug it into soapcalc, just to get the number at 12 (the minimum number for cleaning)?
You can use zero coconut if you want to. A 100% olive oil soap will have a cleansing value of 0, so there really is no minimum. This is why the numbers on soapcalc make for a good rough guideline but should not be considered written in stone. No coconut oil (or palm kernal oil, or babassu) will give you lower cleansing numbers, but it will also give you far less bubbly lather. If you like a creamy lotion-y lather, then that's fine, but if you want big bubbly lather you need something to give you that boost. As I said, if you want to use coconut oil but you are worried about it being too drying, just up your superfat a bit to compensate.