True/false statement for poison damage (D&D 5e)

So I picked up the macro system and started making all my weapon attack macros to streamline the process. For my Ranger, I made some poison arrows which our GM decided that the damage is 2d4 and the fail save is a con save vs. 11. If you succeed it's half the damage, if you fail it's full damage. I've been wondering if there is a way to make a macro statement so that when rolling damage dice it automatically decides if the target takes half the damage or the full damage?  This is what I have so far /r 1d8+[[ @{Roxas J Quarrel|dexterity_mod}+@{Roxas J Quarrel|pb}+2 ]] +?{Poison Arrow?| Yes, [[2d4]] | No, Normal Damage }+?{Hunters Mark?| Yes, 1d6 | No, Normal Damage }+?{Colossus Slayer?| Yes, 1d8 | No, Normal Damage }.  I'm I want the query of if I'm dealing poison damage or not to include that constitution save dictation.

You could do something like: [[ floor(2d4 * ?{Save|Yes,1|No,.5}) ]] to get the damage right.  As for not prompting if you aren't using poison, you'd probably just need to have a separate macro for poison vs normal arrows.