If it you are sure the oil is 80% rice bran oil and 20% soy oil, then you could figure out how much of the blended oil you wanted to use and then calculate 80% of that amount for rice bran and the remaining amount would be soy oil. Make sure that the blend was well mixed before using. Use those amounts in SoapCalc. Try a one pound, unscented batch at first, so you won't lose a lot of expensive ingredients.