"What I would like to know is my coffin corner margin without the benefit of a speed tape."
If I may make a rash assumption that you do not intend to fly at an altitude greater than the FMC predicted 'Max', then you will be NOWHERE near CC and do not need any 'rule of thumb'.
Of course, if you seek to soar with the angels near 'coffin corner', then you need to consider high speed buffet as well.