In the old days you could be DR'ing along in 8/8ths cloud with no nav-aid coverage at all for 30 minutes then pick up an NDB/DME and fix and be legal IFR. VFR same thing you could DR above 8/8th cloud in VMC and just ensure you had ground fixes (gaps in the cloud) every 30 minutes as long as you were above 3000amsl/1000agl. No need for GPS/navaids. The key was knowing that after 30 minutes or more importantly at your destination you could descend clear of cloud. There used to be a small warning note about extended flight above overcast cloud for VFR aircraft.