You can set a decimal tab via code like so.
VB Code:
  1. Selection.ParagraphFormat.TabStops(InchesToPoints(0.35)).Position = InchesToPoints(0.35)
  2.     Selection.ParagraphFormat.TabStops.ClearAll
  3.     ActiveDocument.DefaultTabStop = InchesToPoints(0.5)
  4.     Selection.ParagraphFormat.TabStops.Add Position:=InchesToPoints(0.35), Alignment:=wdAlignTabDecimal, Leader:=wdTabLeaderSpaces