I usually determine which process I will be printing with before I set up the camera, or if I am not sure, expose at least two negatives so that I can fool with the development. I find that the exposure is usually the same and it is just the development that changes with the intended process (either pt/pd or carbon -- with more development for carbon).

One of the reasons I decide which process early is that I use the single-transfer carbon method which reverses the image, and that needs to be taken into consideration when composing.