1. A computerized method of setting a threshold similarity score value for a first plurality of network device identifiers, the method comprising: receiving, at a processing circuit, a first plurality of network device identifiers and characteristic data associated with network activity of each of the first plurality of network device identifiers; receiving, at the processing circuit, a second plurality of network device identifiers that do not appear in the first plurality of network device identifiers and characteristic data associated with network activity of each of the second plurality of network device identifiers; calculating, for each network device of the second plurality of network device identifiers, a similarity score that represents a degree of similarity between the characteristic data for the network device identifier of the second plurality and the characteristic data for the network device identifiers of the first plurality; designating, by the processing circuit, a performance target relating to a factor indicative of an interest in or usefulness of content placed on a webpage, the performance target used to identify a smallest number of network device identifiers of the second plurality of network device users that have performance statistics data relating to the factor, which when aggregated, achieves the designated performance target; designating, by the processing circuit, a threshold similarity score value selected as a starting value for determining a lowest similarity score value that is used to identify the smallest number of network device identifiers of the second plurality of network device users that have performance statistics data relating to the factor, which when aggregated, achieves the designated performance target; identifying a first number of network device identifiers from the second plurality that have similarity scores above the threshold similarity score value; receiving, by the processing circuit, for each of the identified network device identifiers of the second plurality that have a similarity score above the threshold similarity score value, performance statistics data corresponding to the factor related to the designated performance target; aggregating the performance statistics data of each of the identified network device identifiers to determine an aggregate performance statistics data; determining, from the aggregate performance statistics data, that the first number of network device identifiers is not the smallest number of network device identifiers of the second plurality of network device users that have performance statistics data relating to the factor, which when aggregated, achieves the designated performance target; iteratively adjusting, responsive to determining that the first number of network device identifiers is not the smallest number of network device identifiers of the second plurality of network device users that have performance statistics data relating to the factor, which when aggregated, achieves the designated performance target, the threshold similarity score value to identify the smallest number of network device identifiers of the second plurality of network device users that have performance statistics data relating to the factor, which when aggregated, achieves the designated performance target; and setting the adjusted threshold similarity score value to an experimental threshold similarity score value that represents the lowest similarity score value that identifies the smallest number of network device identifiers of the second plurality of network device users that have performance statistics data relating to the factor, which when aggregated, achieves the designated performance target.