Alessandri would have needed just a little over half of the voters of Tomic, the Christian Democratic candidate. Not saying this would have happened for sure, but I've always thought Alessandri would have had a good chance doing so, especially if the electorate was focused on just him vs Allende.
While Tomic was on the left side of the Christian Democrats, it is indeed a fair assumption to make that a majority of those voters would have preferred Alessandri to Allende. It wouldn't be the sort of landslide Frei got against Allende in 64', but Alessandri would have been favored had there been a second round due to a large part of the electorate fearing or distrusting the idea of a socialist government.
Support for the left (Unidad Popular) grew dramatically from 70' to 71' to the point in which a bare majority of the public actually voted for UP during the 71' municipal elections (the high point of Allende), but his coalition was certainly not strong enough to get a majority by the time of the Presidential election.
Indeed, you can also make the argument that a second round would have also shifted the outcome in 1946 (where the two right-wing candidates won a majority, but were defeated by the left-wing candidate with 40%) and 1958 (where Alessandri himself was elected 31% of the vote). But of course, 1970 is the most significant example. We're still dealing with the consecuences of that one.
