I tried various locations for the mirror, to give the impression of a choice in the road to take, while retaining a strong compositional element (the tall tree in the centre). It seems that as you adjust one variable, another goes out of alignment. Here, I remain a little unhappy about the difficulty of seeing the far edge of the mirror, but alternative positions didn't improve this aspect.
The geometry involved here is simple perspective, having a clear idea of where the horizon is and what the metrics of the camera were.