I'm using the standard D&D 5th edition spreadsheet with the Global Damage Modifier enabled. I have come to realise that it cannot deal with double-digit numbers of dice: 10 rolls over to 0 and 11 rolls over to 1. For example, if you roll a crit with 9d6 Sneak Attack damage as a Global Damage Modifier, it will roll 9d6+9d6 as it should. If you crit with 10d6, it will roll 10d6+0d6. If you crit with 11d6, it will roll 11d6+1d6. Changing the damage to roll a combination of discrete sets of rolls will roll the correct number of critial dice. For example, changing 10d6 to 5d6+5d6 will give the intended result.