Click on the audio play button to listen to this quick post on reciprocity correction for the New Kodak Portra 400 film. In a nutshell: double and triple everything beyond 1 second and you should be spot on.
Viva la Revolution- Stephen
Kodak Portra 400 Pinhole Reciprocity Test
Figital Revolution 
Thanks for an interesting article. Any experience with longer exposure times? My 120 pinhole rig is f256, so my indoor exposures are going to run a half hour or so…
Anything past 20 or so seconds is going to take some testing but remember the film has huge latitude so base your exposure on say and EI of 800 or 1000 and that should help a bit.