When you're mixing together commercial mixes, you don't necessarily need to calculate the nutrients assuming each component mix is itself properly balanced. You can never end up with an unbalanced mix when you are mixing together balanced mixes.
If you did want to calculate it, you would do as follows:
Say, for example I wanted to make a mix containing 300g Harry Hamster and 200g Bunny Dwarf Hamster Expert.
First I need to find the percentage of protein (i.e protein per 100g) in both of these foods. Harry Hamster has 18% protein and Bunny has 15.5% protein.
Now I need to find out how much protein is in the quantities of each food I am using. To find a percentage of a given number, divide the percentage by 100 and then multiply that by the given number. Harry Hamster has 18% protein, so 18 divided by 100 is 0.18. 300 x 0.18 is 54. This means that 300g of Harry Hamster contains 54g of protein. In the same way I can work out that 200g Bunny Dwarf Hamster Expert contains 31g of protein.
Now add together 54g and 31g. Now I know that a mixture of 300g Harry Hamster and 200g Bunny will contain 85g of protein. Now I need to find what that is as a percentage of the whole mix, i.e 85 as a percentage of 500. To do that, divide 85 by 500 and multiply that by 100. 85/500 = 0.17. 0.17 x 100 = 17. So the final mixture will have 17% protein.
Then do the same for fat and fibre if you wish.
Alternatively there are calculators that will work this out for you, such as this:
https://onedrive.live.com/embed?cid=...deHeaders=True