Design formula: R = (V_in,min − V_z) ÷ (I_z,target + I_L), sized so the zener still gets its target current even at the lowest supply voltage. At any other supply voltage: I_total = (V_in − V_z) ÷ R, and I_z = I_total − I_L (the zener absorbs whatever current the load doesn't use). This is the standard ideal-zener model (constant V_z, ignoring dynamic resistance) used by most textbook/online designers — real zeners have a small slope resistance that shifts V_out slightly with current; for tight tolerances, check your diode's datasheet. A healthy design keeps I_z comfortably above ~1mA at worst case and Zener power under its rated wattage at best case.