Did you possibly miss that each property that permits percentage values includes a "Percentages:" line it its definition? For example, fontSize says:Glenn Adams, 2 May 2013, 03:31:44
Percentages: relative to parent element's font size
Initial values cannot be modified. However, for properties that are marked as Inherited, like fontSize, one can place a fontSize property on any ancestor in order to have it apply to what is below (unless overridden). Section 22.214.171.124 clearly defines this behavior.
In order to understand the semantics of presentation properties, it is necessary to understand the XSL-FO definitions, which in most of the cases in TTML, are based on CSS definitions.
There used to be more linkage to this chain of references, but some over-eager sanitizing of XSL-FO references prior to publishing the 1.0 REC removed some of these, which makes it somewhat more difficult to follow the thread of definitional semantics.
If you want to propose some particular examples be added, I'm open to adding them. But absent a more specific proposal, I'm inclined to close this issue, but will simply change to POSTPONED.