From the manual: "Measure if the chain slack is around 25-32 mm (1 – 1.25 inches) at the end of the chain guide."
This means +/- 12.5mm to 16mm above and below the reference (centre line), which should be 0 deflection (pull the chain taught on the bottom).
So, 20.5+12.5=33, and 20.5+16=36.5...