Don,
Thanks for your quick reply.
Here is information about the PPEC guide algorithm from PHD2's website:
https://openphdguiding.org/man-dev/Guide_algorithms.htm
It determines your mount's periodic error and corrects it during a guiding session. It works very well with mounts like the CEM25P that don't have a built-in Periodic Error Correction. Here's a thread from Cloudynights with some user reports:
https://www.cloudynights.com/topic/612512-predictive-pec-with-phd2/
Al