This is not really a radio question, it is all to do with the 1 in 60 rule! If the aircraft is at a range of 100nm, and at the edge of the airway (ie 5nm off track), then:

TEA = DIST OFF TRACK X 60/ RANGE

= 5 X 60/ 100

= 300/100

=3 degrees

At 2 degrees per dot, that would give 1.5 dots displacement