Qamets - The Long and Short of it

I've checked numerous grammars and Weingreen seems to me to have the simplest explanation for when we consider the Qamets as a Qamets - Hatuph.

On page 7 of his grammar he has a rule which applies:

"A syllable which is closed and unaccented must have a short vowel."

On page 12 he uses this rule when explaining the Qamets - Hatuph:

"Since the vowel-sign ָ is use to represent both the Qames 'a' and Qames-Hatuph 'o,' we have to determine when it is (long) 'a' and when (short) 'o'. The rule enunciated on p. 7 is here applied thus:--If the vowel-sign ָ occurs in a closed unaccented syllable it must be short and is therefore (short) 'o' = Qames-Hatuph. If, on the other hand, it occurs in an open syllable, or in a syllable which, though closed, is accented, then it is long and therefore (long) 'a' = Qames."