The indicated airspeed is typically inaccurate near the stall, so if you want to factor a stall speed you have to find the table in the POH that converts it to CAS, then factor the CAS, then translate that back into KIAS. Similarly if you want to add some knots, do it to the KCAS.
But, for obvious reasons, the important speeds are already in the POH in KIAS, so I have never needed to do this.