B737-700
Flight Crew Last 90 Days 168; Flight Crew Type 168
Communication Breakdown; Confusion
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance
Procedure; Human Factors
Assigned CADIT SIX Departure. On departure Runway 8R ATL; told by Tower to fly heading 070 at the middle marker.  Turned NAV radio marker volume to almost full high.  Expected to hear MM and see MM emblem on PFD.  Did not see or hear MM indications so Tower said we were past the MM and to turn left heading 070.  We complied.  The only guidance we could find regarding this procedure is on one of the briefing pages; saying to 'use your best judgment.'  Huh?  We were not sure if ILS has to be tuned in to hear and see marker indications and; if so; it will take approximately 45 seconds to find the chart and tune the radio.  If this occurs on the runway at the same time you are cleared for takeoff; this is an unacceptable amount of time.  We could have had the frequency tuned in STBY radio; but ATL changes runways on occasion; which requires F/O to go heads-down at a very congested airport.  Our Company needs to talk to ATL about this procedure.  'Turn left at the 1 DME;maybe; but this turn at the MM and 'use your best judgment' is nonsense.
B737-700 First Officer reports being cleared via the CADIT 6 RNAV via PDC but on Runway 8L the crew is cleared by the Tower to turn left to 070 at the middle marker.  The crew is not able to identify the middle marker and is instructed to turn by the Tower.