Strength of schedule is calculated from the opponents projected fantasy points allowed for the DL 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.
Justin Smith was a productive IDP back when it wasn't cool for 3-4 ends to be, but it looked last year like Father Time may be catching up to the 34-year-old. Smith's 32 solo tackles in 2013 were the lowest total of his 13-year NFL career, and given that Smith has never recorded double-digit sack numbers in his career that decrease in tackles dropped Smith into low-end IDP DL3 territory. Smith's days of IDP-relevance aren't quite over, but his days as a weekly starter in IDP leagues may well be.