The only reason the Glyph o Combust is stronger is because at pull in theory you can have Alter Time, Meta Proc, Trinkets Proc, Hero, Lightweave proc and a pot going. Under those conditions you get a combust doing significantly better damage than at any point later in the fight.
Which kind of surprises me from the Blizzard front as they have repeatedly said how they hate mandatory Glyphs, and that one there is a no-brainer as it is currently when it comes to mandatory.
They should remove that Glyph, it addresses the main culprit causing huge numbers from fire mages at the top of the gear spectrum. We would still get the stars to align on the pull, just won't get to abuse that alignment via a Glyph.
As to crit levels, CM needs to scale inversely as a flat percentage once at level 90 (before then just give them top number). Consensus seems to be 35% crit smooth's out fire RNG. The Goal of CM should be to keep a fire mage at 35% crit until gear handles the 35% plateau, after which it has no value to the mage. Please note 35% is just the number I commonly see thrown around, could be 30 or 25 or 33, whatever Blizzard believes is the balance point.
CM would then bump any fire mage under 35% crit to 35% crit and any mage at or over 35% would receive no bump from CM. Or whatever balance point picked.
Doing that to CM eliminates the stupidity of a fire mage with 50% crit before CM landing at 65% crit after CM, instead they would be at 50% with CM adding no benefit.
Edit - clarity