First we need to define 'Accuracy'.
If we overerestimated, then it's simple - if we spent 1/3 as much time, our accuracy was 1/3. 1/N x estimate = 1/N accuracy.
Underestimating, though, is the complicated part. What I'm going to assume is that if we spent 3x as much as we estimated, then our accuracy is 1/3. N x estimate = 1/N accuracy.
Assuming you want ...