There are many ways to build a tach. It is a frequency to voltage converter. The hardest part is the trigger circuit that interfaces to points, sorting out a single event without point bounce nor loading ignition coil. Tachs take some energy, and the coil primary peaks to about 500V, so you need high impedance, and voltage protection of detector. Once you have a trigger, then use it to trigger a one-shot. That is a pulse width of x time width, at a regulated voltage. The higher the RPM, more pulses per time, higher average voltage. The meter movement helps filter pulses. The regulated voltage adj, pulse width adj ... work to calibrate. Pulse with can't exceed available time, or it will not work at high RPM.
There are many circuits on web for using NE555 timer chip for tach. It is also possible to use a ATtiny85 uC. The uC is a help to skip over point point bounce, by using initial trigger, ignoring bounce period.
Whatever you do, best to bench test. Need a scope, distributor, way to spin it, or set up an ignition with variable RPM sitmulator.
Use adjustable bench supply to carefully evaluate tach reading vs voltage, start low, plot it out.....
When i have tachs, i never connect to coil, i trigger from tigger signal of egine position from optical or Hall sensors. To drive tach i use small inductor, like relay coil, use it like driving coil, and send a 60V pulse to tach. I also have tach in user interface along with adj shift light.
