Thanks 走馬讀人

In your example, it appears the "oo" in words ending with "k" ought to be pronounced as short "u", and others pronounce long "u".

It might apply to most cases, that's good enough for me.

But again an example in "good" & "food", both ending with "d". "oo" pronounced short "u" in "good", long "u" in "food".