What analog designer hasn't had to derive a nonstandard resistor value by using series/parallel combinations of standard resistor values? In efforts to avoid a production trimpot tweak, we can use 0.1 ...

To calculate resistor values, we need to use Kirchhoff's Law: The sum of currents into a node is zero. Considering the node at the junction of the three resistors, we can write: (V IN - V AD)/R1 ...