finding out fractions of ions(different valencies/charges) of an element in non-stoichiometric compounds
What you have done seems to be correct. But you might want to convert those numbers to percentiles. I can tell you another method based on charge conservation.
Let us take  100 oxygen atoms and 98 Nickel atoms into our consideration.
Total negative charge$= 200$
Total positive charge$= 2x+3y$ where $x$ and $y$ are what you have assumed to be in the question . Now Net charge =0.Equate the positive and negative charges.
Also $x+y=98$. Solve them and you will get the same number. This of course has an high degree of
similarity to what you have done