Go into your connector library.  Edit the connector you are using for the slip-on portion of the flange.  Edit the Breakpoints.  If I recall correctly, you have to set the value of the "snap offset" column as follows:

(("Total Length of the Slip-On Flange" - "Setback Length") * -2)

It's been a while since I did this, but I think that will work for you.