1) nerf by 63% (take 100 as the full damage) so we now have 37 damage
2) buff by 20% - 30% which will give you 44.4 - 48.1 damage

This is a 55.6 - 51.9% dot nerf

We should be at 67 damage if they nerfed by 33% .. some A+ maths student in zos thought by adding on the % difference between the actual nerf % (63%) and the nerf value they want (33%) to an already lowered value would give the desired value they want

Edit:

ah yeah seems I wasn’t the only person to notice

https://forums.elderscrollsonline.com/en/discussion/496817/dont-be-fooled-but-the-20-dot-buffs-its-still-over-a-50-nerf-from-live#latest