Solution: Targetdummy
the targetdummy resets its hp bar at a fixed hp% amount, looks like ~10%. only important thing here is, that it resets always at the same amount of %hp.
so, the simple idea is to grab a stopwatch, grab your gear start the timer and dpssing the targetdummy, once you brought its hp bar down and it resetted to full, stop the timer. note the needed time.
now do the same with the gear/build you want to compare with.
obviously the one that took less time has the higher singletarget dmg output.
nice thing is, that the measurement should take more than 20 seconds*, so you have a nice timeframe and meaningful value to compare (if you want to compare different characters, they need to be same level b/c the td adapts to your level)
*dont know how uber builds perform, my current build needs around 40s for a lvl99 dummy.
just to give an example (and by far not a reference), i compared 4 different relics by this way, all values in seconds and maybe +/- 200ms margin of error, depending how good one reacts
42,55 deathchill
42,26 bane
40,55 ignaffar
39,87 iskandras