Get Phase Change Interface Position During Simulation

Dear all,

I am currently a bit stuck with a phase change simulation where I want to evaluate an expression and use the result for material properties.

To be more precise, I am simulating a 2D phase change problem, where I want to use an average effective thermal conductivity. This effective thermal conductivity, however, is based on the temperature at a specific point (x,y = 42mm, 505 mm) and the position of the phase change interface along a specific line (x,y = 0..42 mm, 505 mm). This has to be avaluated during the simulation (not in postprocessing)

Now my questions are:
a) Where should I put the expression? Under "definitions" of "component 1" or under the "materials section"?
b) Can I access the necessary temperatures in this expression directly (s.th. like comp1.T(0..42,505)..) or is it better using a domain point probe?
c) How can I evaluate the position of the phase change interface? Principally, it is the element where the temperature is above (or below) the melting temperature. So I need to go along the line from x=0..42 mm, check if the local temperature is above T_melt and if true, save the current position and finally feed the last position into my expression for the effective conductivity.

Many thanks in advance!

Chris