Couple of points to keep in mind ..
(a) there are other considerations in the certification side of things apart from stall speed and weight. Just figuring a reduced speed might not keep one sweet unless the rest of the Design Standard requirements be checked.
(b) if one is going to do sums with speeds, first convert from IAS to CAS, then do the sum, then convert the answer back to IAS, lest the answer be in error.