bro,what i can suggest is that you mix the salt mix till half of the tank volume around 1.027, NSW is around 1.021.
once the salt mix is mix properly, add NSW to it and measure the SG.
adjust via mixing the salt mix out of the tank (maybe pail) instead of pouring salt into the tank already with salt to mix it.
maybe that will be better (can have some live bacteria from the NSW and better paremeters of ca/mg/kh with the salt mix.