VCL TFont uses negative Size to specify that internal leading on top of each line of text is included in Size.
TeeGrid assigns Self.Font.Size to Tee.Format.TFont.DefaultSize when ParentFont = true.
If parent uses negative Size then Tee.Format.TFont does not work correctly.

I have made a simple workaround for this in Tee.Format.pas:
-added public function to convert between negative and positive Size
class function TFont.GetTeeFontSize(const AWinFontSize: Single): Single; // DM CHANGE
begin
  if AWinFontSize >= 0 then result := AWinFontSize
  else result := Abs(AWinFontSize) * 0.9; // approximately font size - internal leading
end;

-changed following methods to use the GetTeeFontSize when setting internal font size:
Constructor TFont.Create(const AChanged: TNotifyEvent);
begin
  inherited;

  FName:=DefaultName;

  DefaultStyle:=[];

  FBrush:=TBrush.Create(AChanged);
  FBrush.InitColor(TColors.Black);

  FSize:=GetTeeFontSize(DefaultSize); // DM CHANGE
end;

procedure TFont.Assign(Source: TPersistent);
begin
  if Source is TFont then
  begin
    Brush:=TFont(Source).FBrush;
    FName:=TFont(Source).Name;
    FSize:=GetTeeFontSize(TFont(Source).FSize); // DM CHANGE
    FStyle:=TFont(Source).FStyle;
  end
  else
    inherited;
end;

function TFont.IsSizeStored: Boolean;
begin
  result:=GetTeeFontSize(FSize)<>GetTeeFontSize(DefaultSize); // DM CHANGE
end;

procedure TFont.SetSize(const Value: Single);
begin
  if FSize<>GetTeeFontSize(Value) then // DM CHANGE
  begin
    FSize:=GetTeeFontSize(Value); // DM CHANGE
    Changed;
  end;
end;