Proper Water softener water and salt levels.

I make my living as a handyman and one of the little jobs I do for little old ladies is put salt in their softeners. Over the years I've noticed that softeners have the salt and the water at a variety of levels. From nearly empty to almost full. I have asked a number of people (home-owners and sales types) what are the preferred or proper levels. No one seems to know. It seems to all be guess work. I just installed a new softener for a customer and thought I would get the right skinny once and for all. No way Jose! You'd think the manufacturer would tell you. Is it a big industry secret?
All it told me was that the softener itself would set the proper water level. It didn't say how much salt to put in. So I filled it half-way with the water level just above it, about an inch or so.
So what's the answer? Should the salt level be full, half-full, 1/4 full? should it be above the water level, or below it, or even with it? The house I'm living in the tank is nearly full with salt and no sign of the water level. Is that good, bad, or what. I just spent a half hour looking for the answer on the internet and so far I still don't know. If you don't know, is there someone you can refer me too who might.