There always seems to be a lot of confusion about the subject of mineral supplements. Find a mineral that is recommended for your area and forage base, then always read the tags to check for ingredients and recommended consumption. Some complete mineral mixes contain salt. Others don't. I prefer one that does contain salt. Trace minerals are included in most complete mixes, but are not a substitute for a complete mix. Where salt is included in the complete mix, additional salt is not required, but certainly lowers your costs at certain times of the year.
I know a lot of people keep out salt blocks in addition to a complete mineral mix that contains salt. I never understood that. When you do that, you are giving the cow a choice she is better off not having, in my opinion. Some will lick the blocks and never touch the minerals. I never did believe in the garbage about a cow knowing what she needs and eating it. Most cows are like kids---they'll go for the ice cream first, followed by something that is less desirable. And then there's the ones that have different tastes altogether. You just can't ever figure out what they're gonna eat and why. That's why I don't believe in giving cows a choice with something as important as minerals.
It's really hard to figure out mineral consumption if some cows are just licking the salt blocks and others are eating twice as much mineral as they need. Your average is right, but you're not really delivering every cow what she needs. In my opinion, the only way to be absolutely certain that every cow is getting her minerals is to make her lick the mix to get her salt. Also, in my opinion, a loose mix is the only way to go.
I use Evergreen Triple 12 minerals. Contains around 12% each of calcium, phosphorous and salt plus trace minerals, vitamins, etc. As with most minerals, recommended consumption is two to four ounces per head per day. I try to keep the average somewhere in that range beginning about a month prior to calving and continuing through breeding season. At other times of the year, I don't worry about it if consumption falls off some. Not saying this is correct, just what works economically for me.
I have to mix salt with it to keep consumption in that range of two to four ounces per head per day when the weather starts heating up and salt craving increases. Minerals are just too expensive for me to allow overconsumption. When I mix salt, I just use the cheap white stock salt. Not only is it cheaper than trace-mineralized salt, but it is the right texture to blend easily with the mineral I use. The cows are already receiving trace minerals from the mix, anyway. To me, the most important thing in mineral supplementation is not to give cows a choice. You decide what they lick.