Substituting: What to do when you can't wait to create!, there seems to be some confusion about how to use cetyl alcohol with BTMS-25.
Why add a fatty alcohol to a conditioner? They can boost the substantivity of the cationic quaternary compound, which means more conditioning awesomeness for our hair! Fatty alcohols increase the substantivity of the conditioner by adsorbing to the hair fibre as well and encouraging more adsorption by the quaternary compound. Fatty alcohols can also act as oil free moisturizers. To increase this conditioning power, we add equal amounts of cetyl alcohol to match the amount of behentrimonium methosulfate (BTMS) we are using.
If we are using BTMS-25, there is 25% behentrimonium methosulfate in the product, hence the name BTMS-25. We would add cetyl alcohol at 25% or 1/4 the amount of BTMS-25 we are using.
If you have 4 grams of BTMS-25, you have 1 gram of BTMS. Add cetyl alcohol at an equal amount to the BTMS, or 1 gram.
If you have 8 grams of BTMS-25, you have 2 grams of BTMS. Add cetyl alcohol at an equal amount to that BTMS, or 2 grams.
Multiply the amount of BTMS-25 you are using by 0.25 to get the amount of BTMS in this ingredient. Or divide by 4. Whichever way you choose, you need to figure out the BTMS in the BTMS-25 to figure out how much cetyl alcohol to add.
If we have BTMS-50, this means we have 50% behentrimonium methosulfate or BTMS in the product. So if we have 4 grams of BTMS-50, we have 2 grams of BTMS in the product. If we add cetyl alcohol at equal amounts as the BTMS, we would add 2 grams to the product.
Multiply the amount of BTMS-50 you have by 0.5 or divide by 2 to find out how much BTMS is in this ingredient. Add the same amount of cetyl alcohol by weight.
If we have BTMS-225, which has anywhere between 20% to 25% behentrimonium methosulfate, you would want to use 0.8 grams to 1 gram cetyl alcohol if you are using 4 grams. Multiply by 0.20 or 0.25 to find the amount. (I'd go with the 0.20 figure as it's better to have slightly too much than not enough in this case!) Or divide by 5.
After writing all of this, I think the issue might be the name of the product - BTMS-25 or BTMS-50 - might be confused with the name of the cationic ingredient, behentrimonium methosulfate or BTMS? It really is very important to read the end bit of the ingredient - BTMS-25, BTMS-50, BTMS-225 - because these tell you how much BTMS or behentrimonium methosulfate you'll find in the product.
What's an INCI name?
Reading INCI names (about BTMS)
Adding slip to conditioners with fatty alcohols