"...What i do is use soap calc calculator,selecting 90% pure,and then multiply the lye amount by 0.05. As 5% is the difference between 85 and 90...."
This is a good approach to the problem, but may I suggest doing this instead. I think it will give you more accurate results...
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1. Calculate the KOH at whatever purity you choose in SoapCalc.
Let's say you stick with the 90% purity option in SoapCalc. Using the recipe you gave above, the result I get is 110.7 grams of 90% pure KOH.
If your number is a bit different, that's fine. Just use the number you get from SoapCalc.
2. Correct the weight of the KOH needed based on the actual purity of your KOH:
Grams KOH at 85% purity = 110.7 X 90 / 85 = 117.2 grams
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Look at the fraction of 90 / 85 in the math problem above. See how the first number is the purity used in the calculations and the second number is the actual purity you want?
Here's the generic formula so you can use this for any soap recipe:
Grams KOH at your actual purity = (grams KOH based on the
soap calculator's purity) X (Soap calculator's KOH purity) / (Your actual KOH purity)
I hope this make sense!