KOH = 100% pure
90% = 90% pure
All KOH is likely between those two %s
If the KOH you have is 95% (for example) pure, then you can use the above to see how much KOH you need to use to make the LS by putting first one and then the other into the calculator and taking the average of the two numbers to find the amount of KOH for your batch.
But there's more...
KOH is hygroscopic, meaning, it absorbs moisture from the air, every time you take the cap off. (Be sure to recap the container promptly after weighing.) You should have an MSDS that came with the KOH that says 99% (or whatever) pure. If you didn't get that sheet, then it should be noted on the site.
I often use 90% (old school, Failor style) to add +10% KOH, especially if I'm using oils like olive, avocado or butters that are high in "unsaponifiables", cuz I'm OCD about clarity. I also use 0% SF for the same reason. Of course, this means that later on I need to add 20% citric acid solution to neutralize any excess lye.
On the other hand, like many soapers, you may choose to not use 90% and go with 3% SF thus eliminating the need for neutralizing later on, and still have clear soap, depending on what oils/fats or other additives are in your recipe. Lard, for example, makes an opaque LS with a pearlescent sheen (that clears over time).
The only way I know to "test for purity" is to make a batch and see if it behaves as it should. KOH can weaken over time and you'll know when you need to use the 90% option because it isn't as loud when you prepare the lye solution as it was when you first opened the container.
I hope that makes sense. If not, no worries. Experience will clear up any questions.
