Hello,
See this page explaining how the atomic fractions are calculated (Fractions of decomposition products). Essentially each entry has to be normalized to 1 atom.
For example, with LiScVO4, it has 7 atoms. Hence, normalizing it per atom results into Li(1/7)Sc(1/7)V(1/7)O(4/7).
If we look at just Li in 8/21(Li3VO4):
8/21*(Li(3/8)) = 3/21 Li = 1/7 Li
Which is the fraction we have for LiScVO4.
As for the example of Ni3S4, could you explain more what you mean by mixing it at a 0.66:0.33 ratio? What does the reaction equation look like?
I hope this helps somewhat!