I was surprised that Bourn isn't quite near the record. The highest percentage I found was Vince Coleman in 1986. Coleman reached base I estimate 209 times. He stole 107 bases, meaning he stole over half the time. My personal favorite season is Tim Raines, 1981. Raines stole 71 bases in 88 games, a pace equal to Rickey Henderson's record breaking season. The runners with 90+ steals tend to have a ratio between .45 and .52.