I got the response but there are still additional questions regarding different/ambiguous definition of these and related columns in different Teradata versions. Please see the question here: http://teradataquestions.com/questions/48/difference-between-filepres-and-fileprereads
My understanding of this is that FilePres is the logical read, which every read will do, then the FilePreReads is the number of physical reads to support the logical reads when the cylinder is not in Cache. on a cylinder basis.
Thus you could work out the Cylinder Cache hit ratio as
We opened an incident with Teradata and they have responded. I've summarized their response at the teradataquestions forum: http://teradataquestions.com/questions/48/difference-between-filepres-and-fileprereads/67#67