First check the calibration with a KNOWN Shop tach..Lay it over the hood and window and do a short test drive and note differences throughout the Gear range..
If you are comparing it to an old STOCK tach, Be sure you are AHEAD of the STOCK tach filter (If provided) to get accurate readings. This will be on the HEI White wire and resemble a "Noise Cap" usually mounted on the manifold.This will screw up your calibration by at least 1/2 after 1500 RPM's on an aftermarket Tach, and if you have both, don't defeat the stock tach, or your wipe the Amplifier card out.
I Should think the pulse rate would be divide by 8 per revolution, on an aftermarket tach, however divide by 2 is plausible (4 per revolution) Never really thought about it..Next time I bring the spectrum analyzer out, I'll make a point to check..I should think it would be divide by 8 though, as it mimics the output side of the coil...Is The Cyl Switch set correctly on the Tach?
If that's the case, you may need an adapter card for the Tach. Contact VDO for specs.