Deer, cows and some other ruminant animals can sense salt deficiency in their own bodies. That's why they will sometimes go out of their way to seek out salt sources. If you happen to live in a place where other people have salt licks out, your blocks tend to last longer than places where salt is harder for them to find. There are a lot of other variables to consider, as well. I've had really good luck with Ranch House brand and the American Stockman Big 6 blocks. They seem to hold together better than other brands I've used(for cattle, that is. I'm in Illinois, so I cant leave them out for deer).
As far as the Di-Cal is concerned, it contains phosphorous, which is good for antler growth. However, its not particularly palatable, so people usually mix it with salt and molasses to get deer to eat it. You should check with your local feedmill. Any feedmill that does their own pre-mixing should have the ingredients to mix a loose mineral supplement that the deer will love. We mix a mineral supplement for cattle here, and cost for us is around 40 cents per pound. $200 for a 500# batch, isn't too bad at all.