[Traindude]

I only very recently discovered the ruler tool in TSRE, and while it's useful, it can sometimes be a pain in the butt to use:

• It seems that, to measure the required distance properly, you have to click and drag "in one shot" -- IOW, if your hand gets tired, you let go of the mousebutton, or make a mistake, you have to CTRL+Z and start all over again, because
  
• Editing a previously-placed ruler seems very cumbersome. For example, if you click and drag an end marker to extend the ruler, it causes the ruler to effectively DOUBLE in length.

So far, I have not found any decent tutorials on how to use the TSRE ruler tool. Some advice would be helpful!

[xavivilla]

That method is not correct.
Select "ruler tool" and click where you want to start the measurement, press the "T" key and then fly to where you want to measure and click the mouse again. The second post and a straight line that joins both will be shown. In the "Ruler" tab you will see the distance between the posts.