Superfatting soap is done to obtain a more gentle, extra moisturizing silky smoother feel to your soaps.
It used to be superfatting was achieved when extra oil above your basic recipe (use soapcalc.net ) was added at trace.
Superfatting soap is when oils are left unsaponified in the finished bars
Now we have sophisticated tools online such as soapcalc.net.
Whether you add the 40 grams of Castor oil at the beginning or at trace will not change the amount of superfat you have in your recipe.
Superfat is the amount of leftover oil after the lye has saponified completely. Saponification continues to take place after you have poured your soap into the mold. If there is lye still unsaponified when you add your castor oil at trace, the lye will find it and turn it into soap.
SAP values represent the amount of lye needed to saponify a given amount of oil. This has been figured out by soap calculators on line for you already and will not change by adding the castor oil in later.
When you enter the superfat amount into Soapcalc, the program will reduce the amount of lye needed which then leaves your soap with 5% unsaponifed oil. Setting the amount in Soapcalc is all you have to do.
An alternative method is to use a recipe that you have plugged into your soap calculator with to superfat and just add 2% of any sofy or melted hard oil, consider a nice oil that lends to great properties like hemp. Remember to mix it well.