Should be something along the lines of:
DELETE FROM ost_ticket_thread WHERE ticket_id<<EM><s>_</s>'current highest ticket_id<e>_</e></EM>;
DELETE FROM ost_ticket_event WHERE ticket_id<<EM><s>_</s>'current highest ticket_id<e>_</e></EM>;
DELETE FROM ost_ticket WHERE ticket_id<<EM><s>_</s>'current highest ticket_id<e>_</e></EM>;
DELETE FROM ost_ticket_attachment WHERE ticket_id<<EM><s>_</s>'current highest ticket_id<e>_</e></EM>;
Though I believe you only have to empty the ost_ticket_event to reset Dashboard.
I haven't tested all of these, but they should have something from you testing.
I believe the solution is also somewhere else in the threads.