Here's some food for thought, and feel free to check with a higher/knowledgeable authority.
I believe that dimension "a" is for disassembly and setup, not a clearance per se. If you are using the same manual I've got, cts-2518, in the assembly section it states to use spacers of a given thickness or a spec'd tool number under the pressure plate before it's bolted to a surface plate. From there once it's bolted down, the release fingers are adjusted to the specified height. I see this as getting the finger height while the clamping pressure is on.
In the clutch specifications chart under "lever setting data" it lists the dimension "a" and the required spacer or tool number.
In another service manual, cts-2039-a, and the section I'm quoting covers a different style of clutch but the same procedure, it reads: "select spacers of proper thickness from se-2068 spacer plate set and place the three spacers at equal distance under the pressure plate, fig 7. See specifications for the the spacer plate requirement." again dim. "a"
so once the clutch is bolted to a jig or flywheel with the right spacer, then The release finger height can be attained. The adjustment has a finite range, so right along with what Mike said about the rebuild shop; the pressure plate has to "fit" the application.
One of the confusing things with IH service manuals is the information that is similar to procedures that are the same for different styles of components does not always carry over to each component or succeeding manual edition.
If I were building this clutch, that's how I would do it.
It May be that what you are measuring is the clutch in it's relaxed State. But if it's been since readjusted, who knows.
If this is beerthirty b.s.; let me know.