Voltage Divider Equation with Load on output
$$R_1+\frac{R_2*R_L}{R_2+R_L}=500,000$$


This is a step which is incorrect. With a 500k pot, R1+R2 always equals 500k (assuming R1 is the 'top' bit and R2 is in parallel with the load, shown in the diagram below).







simulate this circuit &ndash; Schematic created using CircuitLab

You get the right ratio with R1 approximately 25k, and R2 approximately 475k.

As the load is so much less than the end to end value of the pot, the control is more by R1 acting as a variable resistor, than R1 and R2 potting the voltage down.