The highest single season SLG% was recorded in 2001 when Barry Bonds posted an .863 SLG%. The highest career SLG% is Babe Ruth's mark of .690. The highest possible SLG% you can have is 4.000, although you would have to hit a grand slam in your ONLY MLB at bat to achieve this.
Here is how you calculate SLG % :
Slugging percentage (SLG) = (1B) + (2B X 2) + (3B X 3) + (HR X 4) / ABs
To simplify the formula all you have to do is take the number of total bases and divide it by the number of at bats for the player.
Let's look at Curtis Granderson's 2011 season and specifically his SLG% since he led the team last year in home runs. Granderson's total bases stat equaled 322 in 583 at bats. If you divide 322 by 583 you get Granderson's 2011 SLG % of .552, a very respectable number.