What really makes the math easier is the way you did it 100% wrong. If you don't know the difference between an inch and a square inch you probably shouldn't be doing your own pressure calculations. According to Sheldon Brown, you're running your tires without enough pressure:

http://www.sheldonbrown.com/tires.html

To the OP, YES you have to "measure pressure and stuff". When you're dealing with high pressure, low volume tires, you can not go by feel. Just top it up once or twice a week with a good floor pump w/integrated gauge and you'll be good to go.