Apologies for replying to a thread from the past decade; I have the same question as OP, and the same lack of trust for my pixel-perfect clicking abilities.

I wish to measure the distance between two parallel walls:
- How can I make sure that the *first* click is precisely on the side of the wall?
- Even harder, as there are no guides in that case, how can I make sure this is also the case for the *second* click?

Thanks!