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.
It's happened with very little fanfare, but in 2013 Brian Robison quietly had his third straight solid IDP season with the Minnesota Vikings. Propelled by a career-high nine sacks, Robison finished last year 26th among defensive linemen, the third consecutive year in which Robison has posted at least high-end IDP DL3 numbers. As Ben Goessling of ESPN recently pointed out, the arrival of Mike Zimmer (and his proclivity for rotating his defensive linemen) may cost Robison some snaps this year, and the 31-year-old doesn't have an especially high fantasy ceiling, but Robison remains a serviceable DL3 who can be had cheaply late in IDP drafts.