Hm, so if i'm understanding correctly in a fight like t13, echo would decrease your overall dps due to how much faster the adds die and the increased downtime (waiting for DBs, tera etc), so in that fight it wouldn't be appropriate to just multiply your dps by 0.85. Maybe like 0.88-0.9 if I'm giving a rough estimate.

Another reason why I'm asking is because in pug farms for example, dps with echo is generally comparable to dps in a capable static without echo, meaning not many external factors such as add phase timing etc are affected.