You can also use the "Tachymeter" rule.
Despite its name, this rule measures the distance between two trackmarks. When you add it to the session, a small window appears in the lower left corner of the screen with two dialog boxes, "Origin" and "Target".
Lay two TM, then enter the name of the first one in the "Origin" box and the name of the second in the "Target" box. Press "Enter" after you typed the name of the Target TM.
If the "Origin" TM is pointing towards the "Target" TM, you will now read the distance between the two TMs. Otherwise, click on "Forward": the caption will become "Backward" and the distance shall be displayed.
You can move the TM, and the distance will be automatically updated.