Never, ever steal bases with 60% success guys, regardless of how many attempts thy had in real life. Set them to 0 and be done with it.
Bill James published a study he did many years ago in one of his Baseball Abstracts, that showed 67% was the break even point. (That means if your player went 40-for-60 in steals, his baserunning had no effect to your offense). That study was based on data from years when scoring was much more depressed than it is now. My guess is that the break even point is probably over 70%. When you factor in most sim teams have decent armed catchers, your player should probably be at least 75% before even considering allowing your guys to steal. Of course, if you micro manage, you can flip the setting every game or series, based on the catcher's arm.