If you’re in a partisan race, look for elections with the absolute lowest vote percentage that a candidate of your party received in your district. You don’t care what kind of race, as long as the district lines are the same, because you’re looking to find the diehard partisan voters on your side that you can count on, no matter what.
For nonpartisan races look for candidates with similar backgrounds (e.g. labor versus business candidates) and geographies (e.g. where they live or who they represent) and then find races that were for similar offices during comparable election cycles. Calculate the three lowest races just as you would a partisan race.
Issue or ballot measure campaigns are trickier. Rarely are the exact same issues run more than once before, and often, it will be the first time on the ballot. If possible, use previous campaign results that are as close as possible to the issue on the ballot. This is where local expertise is needed to further refine the number.