It's a given that there will be some level of discrepancy due to all DoTs being estimated, but it seems reasonably accurate to me. Did a few trials of recording a video, then going through the video slowly, adding up all the damage numbers I see by hand (Well, with a calculator). Went all out each time, including all DoTs (Miasma II only with Contagion), auto attacks, Shadow Flare, Raging Strikes, X-Potion of Intelligence, Potent Poisoning Potion... Results are consistently within +/- 0.7% of what I'm getting adding it up manually.