Timing is not a precise science on a car of that age. The actual specification will only be for idle and will depend on the distributor. A 1970 will probably expect vacuum retard to be in place (working fully) for the spec sheet. If I remember correctly you should have about 8deg ATDC with those conditions. That was a spec. I always advanced them atleast 5deg above this.
If one sets the timing to 8deg ATDC and the vacuum retard is defective the car can be 5-12 deg retarded.
The total advance with the vacuum disconnected probably extends to thirty degrees BTDC at over 3000rpm. If the total centrifugal advance doesn't make 30 degrees the initial setting should be advanced to achieve 30 deg BTDC at 3000rpm.
Basically if everything is perfect the specification can be followed otherwise these above criteria should be taken to get the guy performing.
Bosch Master, ASE Master, L1
33 years MB technician