I looked this up a while ago and I was only able to find the numbers for Harvard College. I think you may be getting mixed up with all latin honors, the lowest of which is "cum laude". At Harvard, about 50% receive some type of latin honors. What I read is that all the magna's and summa's together comprise at most 20% of the class. So Obama was at least in the top 20% of his HLS class, assuming HLS follows roughly the same pattern for latin honors as Harvard undergraduate.
BTW sometime in the last two weeks a graduate of Harvard Law School posted on here and said that getting a magna cum laude from HLS is a superior intellectual achievement.
And, in point of fact, all indications are that Obama graduated from Columbia with a notably undistinguished academic record - I’ve read that it was C+.
In point of fact, since he hasn't released his transcripts you really have no evidence that he had a C+ or any other particular average.
Thanks for the update on the latin honors at Harvard. If the 50% figure also includes the Cum Laudes, then obviously I was unfairly downgrading the MCLs.
As for the One’s grades at Columbia, true I don’t have a primary source, but the secondary source I read appeared to me to be reliable. I will see if I can retrace my steps to it and give you the reference for your own assessment.