If you do normaly 500 dps then multiple with 1.15 that will be 575. If you multiple 575 with 0.85 it will be 488 dps but original dps was 500. You need divide 15%(echo) with 115%(overall damage) that will be 13.04%. Then after that you need multiple 0.1304 with 575 that is 74.999 and remove that number from your parse so you will reach 500 dps again. That will tell the number what it would be without echo.