>calculated as VRef+80
This is basically correct. To be 100% correct, clean speed is always Vref30 + 80 (Vref for flaps 30, then add 80 knots), regardless of the flap setting to be used for the approach (ie, flaps 20 for a single-engine approach).
>turning the knob on the TMS
While you can use this knob, there are some limits associated with it. AT is normally set using the FMS.