The tie line between basis point to solid solution. for example, in CaO-CuO-TiO2 ternary system; the projection tie line from CaO (1,0,0) to solid solution type (12.75, 37.25, 50), so how to create an equation from this line.
I don't know that i find out your purpose correctly. I think that, you can create an equation for three or more tie lines after defined X and Y for two equilibrium phase. X and Y may be defined as the following:
X=mass of CaO/(mass of CaO + m of CuO and m of TiO2) in phase of Alpha and Y defined same as X for Beta phase (assumed Alpha and Beta to be two equilibrium phases).