As we know there are many experimental methods for calculating the equilibrium factor between radon and daughters, is there are a theoretical method/s for calculating the equilibrium factor?
The question is not clear. What do you mean about daughters? If you are talking about the ionization series Rn->Rn^+ + e -> Rn^+2 +2e ->….. the problem has been discussed in many books.
The equilibrium constant can be calculated directly from the partition functions which is the product of the translational contribution and of the internal contribution, the last one should be calculated considering the boltzmann distribution of internal levels.
In its simpler form, for single ionization you have the Saha equation.
I suggest you to look at the following book
Fundamental aspects of plasma chemical-physics, Thermodynamics, by Capitelli, Colonna, D'Angola, published by springer.