Adding Ingredients to Micellar Cleansing Water Base

You are correct.

The instruction to mix Encapsulated Micellar Cleansing Water 1 part : Purified Water 10 parts means that in the final product, Encapsulated Micellar Cleansing Water will be 1/11th of the total weight/volume (1 part + 10 parts = 11 parts).

If you want to add other ingredients to enhance properties, such as adding moisture, these ingredients will replace a portion of the water to maintain the recommended ratio of Encapsulated Micellar Cleansing Water.

Your calculation example:
Encapsulated Micellar Cleansing Water 100g
Other ingredients (e.g., humectants) 100g
Purified Water 900g
Total finished product 1100g

In this formula, 100g of Encapsulated Micellar Cleansing Water is 100/1100 = 1/11th of the total weight, which matches the recommended ratio of 1 part in 11 total parts (1 part to 10 parts water = 1 part in 11 total parts).

Therefore, your calculation and approach of using 100g of Encapsulated Micellar Cleansing Water, adding 100g of other ingredients, and reducing the water to 900g to reach a total weight of 1100g is correct in terms of maintaining the concentration of Encapsulated Micellar Cleansing Water in the final formula. The 100g of other ingredients effectively replaces 100g of water from the original amount of 1000g that would have been needed (to achieve a 1:10 ratio with 100g of Encapsulated Micellar Cleansing Water).

However, adding other ingredients to the formula might affect the clarity, stability, or overall performance of the product. It is advisable to test the compatibility of the ingredients before producing in large quantities.