Unless the field is formatted with a monospaced font such as Courier, you can't compute the number of pages from the character count as the different characters have different widths.
A sliding field should work for you though you may have to adjust things a bit to eliminate a page break that splits a line of text in the field vertically.
What have you tried with your sliding field settings and how hasn't this worked for you?
I have been confusing Shrinking and Sliding. Now see that whist you can not tell FMP to shrink a field, in setting up Sliding fields FMP will shrink the fields a well as move them. Up to now have only set "Notes" field to Slide up, the result is that Notes field, when printed out, is still truncated. Looks like i need to set all fields to Slide up and look at page breaks with in FMP Reports.