Rob, The cheapo version: to calibrate a variable speed knob on my mill I used a £9 Echowell Star bicycle speedo from Halfords. Ian (boatmadman) put me on to it. Set it so it reads kph rather than mph, and so it thinks it's sensing a magnet fixed to a spoke on a 1667mm circumference bike wheel. Readings from 0.0 to its maximum 199.9 kph translate as 0 to 1999 rpm. Just to check it against my lathe's built-in tacho (with which it agreed) I stuck a tiny magnet into a cork and the cork into the outboard end of the spindle. Doubtless you could devise something more sophisticated, but I know someone who uses a cork all the time unless it gets in the way of long stock etc.
Andy
