FluentNHibernate mapping; Unable to map double or decimal with scale/precision

I'm working first time with FluentNHibernate, trying to map classes to SQL Express database. In general it works, but I'm unable to map Double or Decimal property types to specific scale/precision. Below shows result for a single property that I tested over and over with SchemaUpdate.Execute. In no case was I able to get it to work.

Would be really helpful to hear some explanation to the mappings that does not work as I expect (2-8)?

// Ok mappings:

1) Decimal: Map(Function(x) x.Balance) >> Decimal(19, 5)

// Mappings "errors":

2) Double: Map(Function(x) x.Balance).CustomSqlType("decimal") >> Decimal(18,0) - Why 0 precision is the default mapping here?

3) Double: Map(Function(x) x.Balance) >> Float , But; when running SchemaValidator after: HibernateException: Wrong column type in FnhDb.dbo.Account for column Balance. Found: float, Expected DOUBLE PRECISION

4) Decimal: Map(Function(x) x.Balance).Scale(9).Precision(2) >> SqlException: The scale (9) for column 'Balance' must be within the range 0 to 2.

5,6) Decimal or Double: Map(Function(x) x.Balance).Scale(9).Precision(2).CustomSqlType("numeric") >> numeric(18,0)

7,8) Decimal or Double: Map(Function(x) x.Balance).Scale(9).Precision(2).CustomSqlType("decimal") >> Decimal(18,0)

EDIT: I include code and hbm.xml (export) for case (4) here:

Public Class AccountMap
    Inherits ClassMap(Of Account)

    Public Sub New()
        MyBase.New()

        Id(Function(x) x.Id).GeneratedBy.Identity()
        Map(Functi