How would one do the math to find that ? I'm curious. I tried earlier, but obviously failed since I found a bigger DPS lost

On a 2.5 mil parse you'll use shadow stride 8 times and a total amount of additional downtime of 2 seconds. Shadow parses about 10.5K DPS, so when you calculate that out your total loss is about 80 DPS off your parse.

It's a larger loss if you're not good at the spec and your APM are low, or if you have larger lagtime with your ping, or if somewhere in there you lose a saber strike due to the extra downtime. Obviously RNG can be a rather annoying factor with this spec too. But the actual DPS loss is only like 80 DPS.