Tach accuracy

70 Duster, 340. I have the rallye dash tach. I haven't compared it to another tach yet but some symptoms point to it being inaccurate, if I am correct it is reading low. I have an Accel distributor, pn 59301, MSD blaster 2 with a Rev-N-Nator ignition box. I read on For B-Bodies only some people having issues with tach accuracy using electronic ignition. I see there are various tach adaptors listed at summit, are these needed to make the factory tic toc tach accurate, or do I have to go to a aftermarket tach? Would like to keep it stock looking if I can. This is a street car, but I would like it reading pretty close.