Hi blueboy1977,
Are you speaking of an inch in height ? I'm asking because it should be a ring - like a wedding ring and no more so 2 mm or 1/15 of inch .
My way of doing: I never girdle, and still they root . IMO, the timing is important . June and July are the best months for that, because both the air-layer and the mother-tree have still time to
put some growth after being trimmed .