There is an RVI conversion that can be done on the Tachometers that make them operate properly with an electronic (Pertronics) pickup…. I did it on my TVR.
My Tach was running at double the actual RPM as well… (oops… yours is running at 1/2 speed…)…
You can read about it here…
https://groups.io/g/usa-tvr/topic/83630746
Rocky