You have to construct a model which counts the total of t2g and eg electrons which contributes to magnetization. This will gives you a magnetic moment per cell that you can transform in magnetization dividing by the volume. You will get magnetization at 0 K. The saturation magnetization is decreasing with increasing temperature. You may easily find this kind of thing detailled in any thesis which deals with ferromagnetic manganites. Mine is in french so I think that it may not help you.