This one's going to be quick.
A lot has been written as of late about the the importance of the efficiency gap, the statistical tool put forth by Stephanopoulos and McGhee that allows us to measure partisan skew in voting in an unbiased way. It's an incredible concept, and the center of the Gill v. Whitford Supreme Court case..
The linked spreadsheet here is structured to guide you through your own efficiency gap math. If you: (1) are curious if your city or state or national legislature is statistically unrepresenative of its voters and (2) there are primarily two parties vying for its control, then this is the spreadsheet for you!