Electronic Tacho

I am thinking of fitting a cheap electronic tacho to my Trident, it has a Lucas Rita ignition system which is wasted spark, so i would presume the the trigger wire would just attach to the trigger/low tension side of a coil, but what i am wondering is could the Rita system be affected by doing this?
and am i going to get RPM x3?
And before its suggested the original tacho drive internals have been removed so would require an engine strip to replace.