BOM How to creat a build

My industry works in precious metal like gold. So there is pure gold which is 24K. 24K is mixed with Alloy to product 18K and 14K.

I created an item called 24K and another called Alloy. What I assume is the best way to create 14K & 18K is to create a BOM like this;

18K BOM:
0.75grams 24K
0.25grams Alloy

Then if I build 10grams of 18K it would use 7.5grams of 24k & 2.5grams of Alloy.

How do I do this in ERPNext. I cannot figure it out.