Are we talking about accuracy or effectiveness? Accuracy is irrelevant if you can't find the aiming point and the woeful results early in WW2 are surely down to the inability to find the target town, never mind the aiming point. Someone out there should be able to quote the sort of CEPs being achieved on the range at the time, but in 1946 IX(B) with its new Lincolns was proud to record an average error (not CEP) of 32 yards for 8 bombs from 20000 feet - step forward F/O Myatt and crew! I suspect 300 yards was nearer the mark in real life.
Navigation without radio aids is/was not easy and AFAIR in the 50s a requirement for classification as a Select Canberra crew was to keep within 50nm of track on astro.