I agree with Irena, I would use a minimum of 5%. But an exact number is a personal choice. I almost always use 6% as my magic number and Irena most always uses 7%. That is one of those things you need to decide for yourself, and the only way to really do that is to try a few different rates and see what you like best for your recipe(s), your local humidity, and your skin.
Those book recipes may already have a certain sf % figured into the recipe (if they list exact amounts of lye/water), but even if they do, when you run the recipe through the lye calculator (and, as Irena said, you should do this for every single recipe you ever make) you can alter the sf to be what you want and use the numbers you get.
_________________ Remember, it's not just that we weather the storm, but how we dance in the rain.
Tammy
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