DnD 5E Empowered Spell - Macro Reroll Once up to Ability modifier (minimum of one)

"When you roll damage for a spell, you can spend 1 sorcery point to reroll a number of the damage dice up to your Charisma modifier (minimum of one). You must use the new rolls." How do I use this case considering my Charisma modifier (minimum of one)? I know only  Special Case: Reroll Once (B,F)  roCP

There's no way to do conditional rerolls of this sort automatically in roll20. If you know you are going to spend a sorcery point ahead of time you can use the Keep Highest roll, something like /roll {@{damage}d6 + @{charismamodifier}d6}kh[[@{damage}]] But you wont know that ahead of time. The only way to do this is manually.  You could sort the dice, so you can easily see which is lowest, and then manually roll an extra die if you choose to spend the sorcery point.