its probably not calcium your lacking its more likely iron deficiency. It can cause thinning and abnormally curved nails. Its called Hapalonychia. I would try maybe some iron pills.
I had actually heard from a doctor a looong time ago that the curl could be from some type of nutrient or vitamin deficiency. Mine do this too but I keep them short so it doesn't bother me to much.
The problem with thin nails though, when I was a kid I used to bite my nails, I always thought they were thin because of that. (Was a really long time ago though).
Have you tried prenatal vitamins? They make your hair and your nails grow. You don't have to take them just because you're pregnant either. I know lots of people that take them that are not pregnant and are not even trying to get pregnant.
some nails are thin like that and do curl under when they grow. there's nothing you can really do about it. its a normal thing so dont worry about it. its just in your genes