He presently has 77 million shares
IBM float will decrease from 980 - 880 million shares over that time. Share re-purchase at $6 billion/year
He will receive over 6 million shares from dividend re-investment. Dividends at 2.6 - 3.0%
He will invest another 1-2 billion during that time ( 5-12 million shares)
From there he could easily own 13% in another 3 years using the same approach. Warren loves buybacks!