1. A method for enhancing signal strength for placing sub-resolution assist features (“SRAF”) with respect to one or more target patterns in a mask layout, comprising: generating an initial SRAF guidance map for the mask layout; placing a first set of one or more SRAF in the mask layout according to the initial SRAF guidance map; altering one or more of number, location and dimension of at least a portion of the first set of one or more SRAFs; measuring improvement in signal strength in the initial SRAF guidance map; and generating an updated SRAF guidance map with enhanced signal strength.