Makes perfect sense and I logged this with OSIsoft vCampus a couple of years ago. It can't be done, I remember one client constantly runs out of characters for a dataset because of they had to put nested calculations in one dataset.

From memory the answer from OSI is that AF is the target platform for such nested calculations and not ProcessBook, you can then access the calculations in ProcessBook via the AF dataset. If you don't have AF then you would need to purchase it, which might be easier than you think with PI System 2010 just been released.

Something to note on this topic is the benefit you will get out of having AF to produce these sort of calculations and nested calculations. The calculations can be re-used to your hearts content and provides a central place to document the calculations (and one place to change them). I have recently done this with a client and not only are the calculations clear to anyone (in the past the person that wrote didn't publicise them), all constants were described or UOM's used and lots of initiatives were generated based on the availability of information. Typically, users will produce something useful but when they leave that usefulness is lost so building them centrally in something like AF is the way to go.