Chief - yes all your analysis there is right - definitely only bet 1/2 unit if betting on rough trifectas.

PS. Does anyone know the rules for the various TAB's if NOBODY has the selected combination? I believe that Supertab pays out on the selected horses in any order (ie. if it was 15-13-11 past post but nobody had this in trifecta they will pay on 11-13-15, 13-11-15, etc)? I could not find any reference on any of the TABs websites to the actual rule in this case.