Strength of schedule is calculated from the opponents projected fantasy points allowed for the DB position. The best rating is +16 which gives the player the advantage against the defense and indicates an easy opponent, the worst rating is -16 which gives the defense the advantage and indicates a tough opponent.
It may seem odd to recommend a nickel cornerback as a potentially valuable IDP asset, but bear a few of things in mind. First, NFL teams nowadays are spending more and more time in their nickel defense. Second, Antoine Winfield has shown the ability to be very productive from an IDP perspective in the past, due to his tackling ability and willingness to get dirty against the run. Lastly, Richard Marshall showed with the Carolina Panthers that a nickel corner can be fantasy-relevant. Just sayin'.