How to calculate the thickness of the ice crust on a frozen ocean planet/moon?
Based on assumptions, if you are ok with approx solution then you can solve heat transfer equation in steady state. 

Heat transfer equation

Ignoring the time - long time has passed to attain equilibrium. 

Heat flux assuming outward and hence internal ocean would be sum of all internal heating mechanism.

Easiest would be if somehow you can come up with surface temperature value. Then you can solve conduction equation through ice for temperature and  weight for pressure and just see at what temperature and pressure ice will go into liquid. Simply asking what is ice layer thickness to maintain a internal heat flux and given surface temperature.

You need one boundary condition. Other way would be little harder, starting at centre with outward heatflux. You need to know both phase and composition since density and cp both changes with these conditions.