Approximations and Rules of Thumb

Pi seconds are a nano-century.  (pi ~ 3600 * 24 * 365 * 100 * 1e-9)

Maximum current on PCB trace: 500mA per 10mil 1oz trace.

PCB 1oz resistance: 0.5mOhm per square

Parallel resistance:  R1 || R2 ~ R1 (1 - R1/R2), R1 << R2

Quadratic equation:  

Divider tolerance: