I'm using Slic3r Ver.1.3. I found that there exist a presetted gap in between the raft layer and object layer, excluded 0 the minimum gap I can get is 0.22mm for an 0.1mm object layer and 0.12mm for an 0.2mm object layer. It will be fine if gap is smaller than object layer height but not vice versa, due to not enough adhesive force the edges of the printed object all warped especially when printing ABS. I just wonder why they let you change the contact z distance but reserve the critical range presetted. By analysing the G-code generated I obtained the above data and listed in the attachment.