I fitted a similar if not the same unit to my C2 lathe and it works well although I didn't use a magnet as depicted in the photo , I found it worked well just sensing a gap in a metal disc but yours may differ .
I tested the spindle RPM with my optical/ laser tachometer and found there was only a few RPM difference between the two readings- luckily as I bought 4 of them and I think they were aroud $20 au each with free postage .
Ian