Wired up to the white/black wire from the coil and it acts really strangely, it's really laggy and inaccurate. I emailed them and they replied around 10 mins later with this:
im afraid minis always create problems with our rev counter. It is mentioned in our gauge descriptions so if it was missed form your description then my apologies for that. Even without wasted spark etc, minis still cause slight compatibility issues that we cannot resolve.
Anybody know why this might be? I quite like the tach and would like to get it working if possible.