The DataType Attribute is a Validation Attribute. You need to do that using the ModelBuilder.
public class MyContext : DbContext
{
public DbSet MyClass;
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity().Property(x => x.SnachCount).HasPrecision(16, 3);
modelBuilder.Entity().Property(x => x.MinimumStock).HasPrecision(16, 3);
modelBuilder.Entity().Property(x => x.MaximumStock).HasPrecision(16, 3);
}
}