Spectrum assignment systems and methods include, for an optical network with a plurality of links with optical spectrum on each of the plurality of links managed utilizing a flexible grid, obtaining a set of S services which require spectrum assignment on various links in the optical network; partitioning the set of S services into a plurality of groups G, wherein each group has one or more of the S services that are disjoint from one another based on occupancy of links, and wherein the plurality of groups G are selected based on fill which is a number of links occupied of the plurality of links, standard deviation, and width of spectrum; determining a sequence of the plurality of groups that minimizes useless spectrum, the useless spectrum being slots of spectrum unavailable for new channels; and assigning spectrum to the set of S services based on the determined sequence.