I calculate it in terms of moles. From previous examples, I calculate N2 and O2 with a 0.21 and 0.79 ratio. So O2/(O2+N2)=0.21 and N2/(O2+N2)=0.79. Then with stiochiometric conditions, I have a ratio of 9.52:1 for Air to Fuel based on the general equation. With the equivalence ratio given, the only unknonwn in the equation is the Fuel_actual. 

Equivalence Ratio=Z/(x+y) * 9.51/1 where z is the fuel_actual, x is the n2_actual and y is the o2_actual. I have 3 equations (top 2 mentioned in previous paragraphs with n2 and o2 and the third is equivalence ratio eqn) and 3 unknowns. I also know that z+x+y=1 bc it is expressed in mole fraction. I then solve for one variable to find the other variables. Is this approach correct?