I used PDMX's spindle sensor, but see now that it's discontinued. You can just use a proximity sensor for this, possibly not use a metal disc with a cutout on the spindle, but a tab. But that might just be the NO/NC part reversed, I don't know. I had big problems with interference with mine for awhile, spindle would read in the millions of RPMs, Bob Campbell suggested I use a small capacitor across the signal lead and to ground. That fixed 90% of the problems, last one was I had the signal wire going into the power supply/ Gecko/ BOB box, and the wire was close to the torodial transformer. Tywrapped it up away from the trans and all was good.
DaveA.