Hi PI,
With 0.18mm line you can use up to a 0.40mm leader without a step down.
Check the Spyker leader knot in the knots and rig section:http://www.sealine.co.za/view_topic.php?id=41669&forum_id=44
I personally use the 0.28mm Reflector or Camo for weights up to M2 and the the 0.35mm for Bigger weights.
The same knot can be used with a braid leader but wrap the braid 8-10 times around the double nylon.
To answer the actual question is where a leader is made with a main leader of 1.6 times the length of the rod, lets say 0.35 then 1m of 0.28 then 1m of 0.22 then your mainline.
I find it to be to much hassle and just more knots to hook on a guide etc so I go straight from mainline to leader, if you rod guide spacing is right your leader won't bunch up and catch the guides