Ditto to Minnflyer. get the red plastic centering tool. I've only built two kits, the first I got half way through slotting with a #11 and became very frustrated. The slot machine made slotting fun!

I have found that you do need to plan your slotting a bit as the slot machine will not get into tight corners (next to wingtips or fuselages). I just make sure to cut slots in TE pieces a little earlier than the plans may call for.