How do I macro damage and half damage on a save ?

Just to save a bit of game-time math for the players, I'd like to be able to show what half damage (rounded down) is for those who save against certain spells. A typical macro I use would be... /emas The floating githyanki projects a huge bolt of lightning 100 feet long and 5 feet wide doing [[6d6]] lightning damage. Dexterity save DC 19 for half damage.

You can't without using the API. There is no way to save a roll to use later in a macro.

Your /emas text should be enough for the characters, really if they can't handle the arithmetic of dividing numbers by two, you should probably set a drink limit for players. I use the AD&D2E rules which total out at about 9,000 pages, if I can even sort through that, I assume players can do simple arithmetic. If they were using physical dice they would have to add them up.