Sounds like a classic 'met exam question'! I just put the seatbelt signs on.
IIRC, the 'classic' jetstream (greatest temperature shear) is found in the warm sector, just below the trop, ie in the 'top corners'. I cannot at the moment 'explain' your 'rule of thumb, but maybe the secret is in there somewhere? Falling temperature does - paradoxically - IIRC, indicate you are moving into the warm sector with the higher trop. Bear in mind CFS was some xxx years ago and my books have long been 'heaved', so that could all be gibberish
Paging a met expert... paging a ....................