Most salt mixes make that estimation using treated tap water which contains calcium.
When we use RO
water, which you should be doing, there is no calcium other than whats in your salt mix.
Get a calcium test kit and a bottle of Kent calcium supplement. Get your tank up around 400. Test your change water to and make sure its around 400 before you add it to the tank on water change days and your good to go.
Monitor things from there. You may only have softy's as far as coral but other things in the tank will consume calcium. Your Coraline to name one.
You may want to start experimenting with a kalk
drip to before you get to far along stocking your tank. It took me about a week to figure out just what I need to drip and how strong the mixture needed to be but after that is figured out it maintains my calcium and alk
levels with no further supplementation.