The current through the load at saturations is I_c = VP/Rc. The base current must be I_b = I_c/Beta. Transistors have a varying gain so we want to use the minimum beta value, to ensure saturation.

Rb = (VP - V_BE) / Ib = (VP - V_BE) * Beta / Ic = (VP - V_BE) * Beta * Rc / VP

Base resistor calculation for saturation