We assume the synthesis routes to all target chemicals have already been determined prior to scheduling. These routes can be determined by expert chemists or predictive chemistry tools such as ASKCOS. Algorithms such as SPARROW can be used in pre-processing to downselect routes, e.g., to maximize common intermediates. These routes are represented as a reaction network -a bipartite acyclic directed graph in which nodes are either chemicals or chemical reactions. The production or consumption of a chemical by a reaction is depicted by directed edges in the reaction network. We further assume the yields of all reactions in the reaction network can be conservatively estimated or are known. Given the desired quantities of target chemicals, the quantities of other chemicals are determined by traversing the reaction network in reverse. The quantities are often required to estimate operation processing time; for example, the duration of a solvent removal step depends on the amount of solvent.