cjam,
no, in actual practise it's "10 to 20 feet below when seeing 300", and that's good enough for real life. Three significant digits are not required. You can safely ignore the academics.
I didn't quite understand what you asked about in post #4 in the thread? You'll be 28' below GS at the threshold if you keep your GP antenna at 50' - is that the answer?
Regards,
/Fred