What is the proper Co2 pressure for a half barrel keg?

A good 15 gal keg pressure for regular American beers served at typical temps 32-35 F is 8-10 psi. For a pony keg, try 4 lbs and see how fast the beers is pushed out. I know 8 lbs is too high for pony kegs (7.5 gals)

It depends on the style of beer, and on the temperature the beer will be carbonated at (not necessarily stored at). The amount of CO2 desired is expressed in volumes of CO2 where 1 volume = 1 gram of CO2 dissolved in 1 liter of beer. Desired volumes can range from less than 1 for higher gravity and especially English beers, to maybe 4.5 for a fizzy lambic.

This calculator- http://www.tastybrew.com/calculators/carbonation.html

can help you get the right carbonation for your style and setup.