I think the ‘grabb oakes’ measurement should always be the same, as the Basion should always retain the same relationship to the Odontoid (C2). So, if your measurement is changing maybe that’s a sign of instability too. Here’s another post on further defining Grabb-Oakes:
The cine in flexion, is my idea, I don’t think it’s standard.
My eye’s are shutting, so I’ll have to go to bed now, sorry!