Everyone who has used Revit often uses temporary dimensions, and  even this simple tool has a few tips.  To activate a temporary dimension simply select an element on the screen.  Sometimes temporary dimensions don’t immediately display, this can happen if the element selected are being monitored, locked etc.  If this is the case look for the activate dimensions button in the options bar.  Once the temporary dimensions are available the middle grip on the witness line can be selected(called the move witness line grip), and it can be drug to another object.  Another option is to simply click the witness line which will cause it to cycle from one face of an elements, to the center, or to the other face (this works for some, not all elements i.e. walls).  Input can also be entered once the temporary dimensions are activated, however not everyone knows formulas can also be input.  For example if an angle is activated and the rise and run is known but not the angle, then entering  “=atan(rise/run)” or  “=atan(4/12)” will calculate the angle and will adjust the graphics accordingly.  Another temporary dimensions tip is to start dragging one end of an element, then start typing in a number.  This will automatically activate the temporary dimensions and the entered value will be applied.