QS is an abbreviation for Quantum satis and simply means 'Add as much of this ingredient as is needed to achieve the desired result, but not more.’ 

It is a term used in pharmaceuticals and food manufacture mainly, but you may have come across it in sample formulations given by a supplier.

If we take the example below:  

What you would need to do when deciding what to put in place of the 'qs' is read the guidelines for the maximum use rate.  

In the example above we are missing the amounts for the preservative and the water. In terms of the preservative, you have to follow the manufacturer’s advice; the range might be between 2 and 4% in which case you add the percentage within this range in the ‘preservative’ column.

Similarly with a perfume, you only need to use enough to give the desired scent.  With water, that can be used up to 100% but for obvious reasons, you put in enough water to the formula to make the whole formula calculate  to 100.  

Now it may not stop there, you may have to make an educated guess if you are not sure about an ingredient.  I got a sample of a fragrance and the supplier suggested the use rate as 1-2% - I decided to put 1.5% of said fragrance which was way too much; in the next batch I knew to reduce it significantly, in this case to 0.2%.

In sum, what you replace your 'QS' with may change after making the physical product and doing your tests.

So there you go, that was pretty straightforward!