It's pretty much the same thing as Target Self counting as Last Target. Currently when I cast mini heal from CTRL+B, I press my Last Spell macro hotkey and it casts... mini heal! It should be like this: I click on a spell icon in my screen (Poison Field). I cast a mini heal from hotkey and then I use my last spell macro hotkey -> casts poison field. In short: using the Last Spell macro should always trigger the spell which you've casted last via clicking a spell icon.