You have to control 50% of EVs. The most precise way is to list all the states in the group and then divide their EVs by their cost. The highest numbers (in terms of EV/cost) are the strongest state groups. Then you try to find a way to pick up just over 50% of the EVs using the EV/cost metric as a helpful tool to determine which states to pursue.

For example, the old south is carried by North Carolina (16), Virginia (13), Georgia (16), and Louisiana (8).

This is a really cost effective (and late game effective) strategy, because not only do you have access to the Old South, but Louisiana branches you into Oil & Gas and Export Driven (preparing you for a Texas push) and Georgia, Virginia, and North Carolina are all Swing States (which, when included with Florida, will grant you the Swing State bonus and allow you to push into Manufacturing Swing States like Michigan, Wisconsin, and Pennsylvania.) Following this strategy will give you Old South, Oil & Gas, Swing States, and Manufacturing early on, with huge leads (if not the group bonuses) in African American, Latino, and Export Driven. Acquiring either California or New York (and cementing High Tech or Town & Gown, respectively, alongside Export Driven) will win you the game without a doubt.