How Many of US to Elect A President?

Figuring that there are about 307,000,000 people in the Unite States;  
estimating that 225,000,000 us are over 18;
Understanding that President Barack Obama was elected with 64,000,000 votes;
I calculate that we can elect a president with less than 29% of us of voting age.


Please correct my facts, figuring, and calculations