I don't have the Seneca adapter for the DF700 yet, but if my memory serves me, in one of Paul Marcel's videos on the DF700 or the Seneca adapter, he recommends keeping the sleeve that is shipped on the DF700 and cutting it so that it can be used as a depth stop when using the smaller bits.
I hope I understood your problem correctly and this helps; I just received my DF700 last week and hope to put it to use for the first time later this week building some doors.