There are a couple of different ways that people do it......
One way that some do it is to type your recipe (without your super-fat oil) into SoapCalc at 0% super-fat, hit the view/print button, then jot down the lye amount it gives you. ....
Next, go back to page 1 on Soapcalc (where all of your original recipe amounts should still be), and change the super-fat % to whatever % you want it to be for your finished soap.....
Then choose what fat you'd like to super-fat with and add it to ingredient list, and then type in a random amount for it, like 2 oz or whatever (it really doesn't matter at this point)......
Next, hit view/print and look at the lye amount it gives you. Does it match the amount you jotted down earlier? It may or may not. If it's lower, go back to page 1 and increase the amount of your super-fatting oil until the lye amount on page 2 matches the amount you had jotted down. Or if it is higher, reduce the amount of super-fatting oil until the lye amounts match. That will tell you how much super-fat to add.
Another way that people calculate the superfat (which is more accurate, actually) is discussed here: http://curious-soapmaker.com/superfat-vs-lye-discount.html
IrishLass
Thank you! I had read that article, but it made my head swim!! I think I will try the lyecalc method that you gave.
I read one article where a soaper said that she added a tablespoon of sf oil per pound of oil after cook. I don't know what %sf that would amount to...she did not say...
