Inverse formula for parallel resistance
Easiest way to catalog and do this quickly is to stop using resistance values and use conductance instead. 

Mark the resistors you have with their conductance value and then add those to  get whatever conductance value you want.

e.g. 200k = 5uS (S = seimens not seconds) 

=>  1Meg = 1uS , and 250k = 4uS 

=> 1M||250k = 200k

People seem to forget that the traditional parallel resistor equation does exactly the above.


That is; convert each value to a conductance (\$G = 1/R\$), add them together, then convert them back (\$R=1/\Sigma G\$).