Your chance of successful meditation currently is:
MedSkill / 200 + CurMana / MaxMana
Note that values get truncated to whole percentage points.
Assuming skill of 120 and max mana of 146, then your base chance of success is 120 / 200 = 60% if your mana is 0. If your mana is 10, then you get the base 50% plus a 10 / 146 = 6% bonus, for a total chance of 66%.
When your skill is at 120, you have 100% chance of meditating when you're at 40% mana (which would be at 59 mana if your max mana is 146).
The meditation success chance formula was last changed around August or September of 2007. The old formula was 30% + (CurMana / MaxMana). I'm unable at this time to delve further back in code changes to see what any prior versions of this might have been.