So you have a FRS mk1, in which the tacho reads 1/2 ?
Hard to see how this can happen with the correct tacho, the reason why they read 1/2 is because they are for a conventional, ie non coilpack ignition system.
So the std wiring will be ok for this.
If I assume you are using some aftermarket tacho, most have switches to suit different cylinders amounts, so switch to different setting will sort it.
I don't know if this will work but is something I thought about before, and that is to take a reading from BOTH side of the coil negative, each through a rectifier diode, ie 1N5408, this will then double the signal, as the std coilpack tachos only sense one side.
tabetha