When you say "law", I take it you are from CA and are referring to the minimum 3400 grains per lb salt efficiency they want. To my knowledge, they are the only state with the "law" BUT, anyone selling softeners should be sizing and setting them up for the best salt dose efficiency.
I did mean that they measure the hardness and regenerate accordingly.
No one should use an "estimate" of the hardness, iron, manganese etc., all of those parameters should be set according to accurate and current data. Otherwise there's little chance the softener will/can work correctly.
Yes I do, they are called immediate regenerated twin tank models; I have some 5-6 different versions. They are sized and setup the same as a two tank model, using the same current and accurate data, and then based on the SFR and other 'things'. Very few homes need a twin tank softener due to not having an hour and a half during the night when water isn't being used so a two tank model can be used.
Gary Quality Water Associates