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The saponification value is the alkali needed per gram of oil. Sodium hydroxide values are commonly quoted as grams per gram; potassium hydroxide requires 1.403 times as much because of its higher molar mass. Technical alkali is 90-99% pure, so the weighed quantity must be grossed up.
Alkali requirement
Pure alkali = oil mass x SAP value x (1.403 for KOH); actual = pure / purity
Sodium and potassium hydroxide are severely corrosive and generate heat when dissolved. Use eye protection, gloves and ventilation, and never add water to solid alkali.
Because it depends on the average molecular weight of the fatty acids. Coconut oil with short chains needs more alkali per gram than olive oil with longer chains.
Calculate each oil separately and sum the alkali. Using an average SAP value across a blend introduces error, particularly with coconut or castor oil in the mix.