As shown in the picture, I got 1265 burn damage.
The damage percentage is 1265/(1265+7187), which is about 15% instead of 3%.
Perhaps this is due to 6 stacks of brimstone providing extra 12% damage?
Furthermore, I received another about 6% damage after this burn damage and only had 6770 hp when my turn started. I am not sure where that damage comes from.
The next burn damage was about the same percentage, but I missed the timing for a screenshot.