1. A computer-implemented method comprising: receiving, by a computing system and from a computing device, a request to target a digital content item for display by user devices in a target geographic region, the request identifying multiple computing devices that are located within the target geographic region; obtaining, by the computing system, a targeting accuracy for the target geographic region, the targeting accuracy for the target geographic region representing a determined probability that a particular computing device is located in the target geographic region; identifying, by the computing system, a plurality of groups of candidate geographic regions that each include: (i) the target geographic region, and (ii) another geographic region that has been determined to be similar to the target geographic region based on an analysis of data associated with the multiple computing devices that were identified by the request received from the computing device; obtaining, by the computing system, a group targeting accuracy for each respective group of candidate regions from among the plurality of groups of candidate geographic regions, the targeting accuracy for the respective group representing a determined accuracy regarding whether a particular computing device of the multiple computing devices is located in the respective group of candidate geographic regions; selecting, by the computing system, a particular group of geographic regions, from among the plurality of groups of candidate geographic regions, based on the particular group of geographic regions having a group targeting accuracy that is greater than the targeting accuracy for the target geographic region; and providing, by the computing system and for output on a target interface of the computing device, data indicating that the particular group of geographic regions was selected from the plurality of groups of candidate geographic regions, the targetinterface being configured to receive user input to add the another geographic region of the particular group of geographic regions to the target geographic region to which the digital content item is to be targeted for display.