[D&D 5e by Roll20] Advanced use of Global Damage Modifiers

Hi, I'm just wondering whether there is any way to use Global Damage Modifiers in advanced fashion. Specifically, I am looking for a way to write a Global Damage Modifier that duplicates the damage dice of whatever attack it is attached to. For example, if the attack deals 4d6 damage, then the Global Damage Modifier I want to write should also deal 4d6 damage. If the attack deals 6d8 damage, then the GDM should also deal 6d8 damage. Is this possible?

Not possible from just a macro. You can put an inline roll in the global mod field, but there isn't a way to make that inline roll reference dice that are inside the macro as it rolls it. And any reference to a field on the character sheet would just reference that one field and wouldn't be able to change on the fly. You could put a query in there if you wanted to choose which damage. Other than that you would need some api script probably.

I'm not aware of a way for a macro to do this automatically, though the api might allow it through power cards.  I have never used power cards, so am no help there. As far as getting something similar with macros, some of the fancier sheets have a place to enter a secondary damage roll, which would allow you to just mirror the main damage by duplicating the damage roll there.  You could write a macro with a drop down selection with all the different dice combinations, and have that query repeated on another line for your global damage.  Since a repeated query takes the first answer and applies it to any identical queries after, you would only have to answer once.  This seems like it would be a really horrible macro to write and you would have to answer the query every time you roll damage.  I really dislike queries on something you roll a lot. Another thing you could do would be to set up each character with an an attribute for each attack they use.  So, if "attack_a_damage" is 4d6, you could reference @{attack_a_damage} twice in your macro, once labeled as attack damage and once as global.  Maybe one of those will spur some creativity if nothing else.  Good luck!