Here is my 7 month experience with my Pro500 PID and my opinion.
I tried and tried to resist the group-head thermometer.. Save yourself a bunch of guessing and headache. Order Eric's thermometer. My opinion is that no HX manufacturer today should sell this design without giving the user some full time monitoring of, at least, the water temp leaving the screen. I even wrote to Profitec in Germany about this issue.
My machine seems to operate different than the one tested by WholeLLove. In theirs, they set their PID to 252 and could pull 5 back-to-back shots, 2 minutes apart and temp was within 1 deg of 200. I don't get that. And I think it shows the difficulty in designing a thermosyphon system with a small hole and expect all machine tolerances from unit to unit to be exact enough to get identical performance. I wonder if unit variances might be significant.
In my machine I set the PID to 256. The group idles at about 200F after about 45 minutes. With no flush, the first pull is a little hot and rises to about 205 and settles down to 200. But the actual brew water will be different because of the smoothing effect of the thermal mass of the E61. Only a SCACE could tell what that brew water temp is. What I find is that after each pull, my next idle temp is down 4-5 degrees. So after the first shot, it is idling at about 196. I pull second shot and it goes up to about 202, ends about 199. Likely perfect. But now idle is down to about 191.. I need to wait about 5-8 minutes to get idle back to 195 or the next pull with be sour. Without the thermometer there is absolutely nooooooo
way you can tell this. I had 5 guys over for espresso and the #4 and #5 shots were much worse than #1, #2 and #3.. At that point I had to do about a 15 minute wait as my idle temp was below 190.
I also found in part of my workflow was causing an issue. I was removing the portafilter after each shot and cleaning and leaving out until next shot.. This added to the cooling effect. If I leave it in until I am ready to load/pull, I think the group stays hotter. I was almost to the point of sending my machine back because I could not get consistent shots. But the thermometer saved the machine.
I don't find flushes are necessary. If I am only doing one shot and then shutting down, and since my first shot can run a bit hot, I might do a 4 second flush and then wait for the idle temp to drop a bit and then pull the shot. Again.. how on earth could you tell without knowing where the idle temp is. If my first shot is a milk drink, I leave it hot as bitter is better with milk.
When I contacted my sales guy, he said it is not unusual in HX machines for the brew water to vary over 10F degrees!.. Well.. I don't think that can be true.. but it shows that there is not exact knowledge of the characteristics of these HX designs. I still love my machine and it is fine for my learning journey for quite a while.. but identifying where the brew water temp is impossible without some type of measurement device. I tried all the foam cup and thermocouple devices and they were fine, but tend to only allow a single point measurement; and aren't suited for easily determining what state the thermosyphon water is at.
Be careful of very short flushes that you may use to clean the screen. I've heard that a 1 second or shorter flush could introduce air into the thermosyphon loop and then you'll have issues with brew water temp as well. (a long flush tends to get rid of air). All part of learning about the HX design.