Getting RTTI information inside a Helper class

RTL
1

I have a multiple classes TDecimal3, TDecimal5, as below etc. I want to create a generic helper class that will return 3 for TDecimal3 and 5 for TDecimal5. Is this possible?
type
TDecimal3 = string[3];
TDecimal5 = string[5];

TDecimal3Helper = record helper for TDecimal3
procedure GetSizeandPrecision( var piSize : Integer; var piPrecision : Integer );
end;

{ TDecimal3Helper }

procedure TDecimal3Helper.GetSizeandPrecision(var piSize, piPrecision: Integer);
var
lsString : String;
LContext: TRttiContext;
LType: TRttiType;

begin
LContext := TRttiContext.Create;
LType := LContext.GetType(TypeInfo(Self));
ShowMessage(LType.Name);
LContext.Free;
end;

{ TfrmTestHelperClass }

procedure TfrmTestHelperClass.btnTestDecimalClick(Sender: TObject);
begin
TestDecimalInfo;
end;

procedure TfrmTestHelperClass.TestDecimalInfo;
var
ldec : TDecimal3;
i,j : Integer;

begin
ldec.GetSizeandPrecision(i,j);
end;

2

Okay I gave up and instead made a helper per Decimal Type eg:

TDecimal9_2Helper = record helper for TDecimal9_2
private
function GetAsValue: Extended;
procedure SetAsValue(const Value: Extended);
public
const
DecimalSize = 9;
DecimalPrecision = 2;
property Value : Extended read GetAsExtended write SetAsExtended;
end;

TDecimal9_6Helper = record helper for TDecimal9_6
private
function GetAsValue: Extended;
procedure SetAsValue(const Value: Extended);
public
const
DecimalSize = 9;
DecimalPrecision = 6;
property Value : Extended read GetAsExtended write SetAsExtended;
end;