It is best to over or underexpose a negative film. Pushing or pulling is not the best answer. And, the slower speed films are best for over or under exposing. I have exposed Portra 160 at 200 and 400 with good results and I've exposed Portra 400 at 800 with good results, all with no push.
I've posted the color chart exposures here.
For the best results, exposue Portra 160 at 100 and 400 at 320 though.