Your calcium level is fine for your snails. I have Instant Ocean which mixes pretty low for calcium. I stopped dosing calcium because 1, I don't have anything that really hordes calcium 2, I wanted corraline but I decided to let nature take its corse and 3, I got a skimmer and I've been having a little trouble with overflow so I didn't want to make my situation worse.
Snails don't need as much calcium as some corals would need. I think my calcium is between 300-350 but I can see my astrea snails growing thier outer shell right now- its looks looks pale white flesh.
But if you really want calcium, you could build a gravity fed auto-drip system to drip kalk
. Or you can get Turbo calcium from Kent. I think Seachem makes comparable products but I don't remember the name off the top of my head.