Ankit, kindly refer to our articles on FeO activity in slag and you will find some relations there and a bunch of references we cited that may help you in what you are looking for. 

I hope that could help. 

Conference Paper Effect of Water Vapor on the Activities of FeO And MgO in Sl...

Article Effect of Water Vapor Content in H2-H2O-CO-CO2 Mixtures on t...

Thank You Yousef. I am already aware of the relations and other aspects of activity determination. MY question was relation between FeO and FeO1.5 in steel-making slag. (practical one!). As in your model too Park and Lee's model has a gamma FeO1.5 term. generally we simply put it as zero if FeO term is present separately. Because, during course of reaction some FeO- FeO1.5 equilibrium is an important factor in determining  the oxidation potential of the slag.

I see. You are looking for just a pure empirical relation between the two phases?

Yes. or some relation using the oxygen potential. The problem is that in literature we have plenty of data using FeO or FeO and FeO1.5 but in industrial data available has Fe total in their analysis. I may looking for something to bridge the gap.

The relationship between FeO and Fe2O3 contents of the slag, you can see in reference: "Effect of bottom stirring and post stirring on the oxygen distribution between metal and slag at the end of the BOF process", Eberhard Schurmann at all, Steel research 56 (1985) No. 8. Unfortunately, I have not this paper in electronic form.