I am a little confused about the concern with knowing the exact temp at the group or wherever in a DB -- to me it's like worrying about the exact size of the grind in microns. The important thing is repeatable results and predictable adjustments. That's the main advantage of a DB -- the temp can be set and will basically be pretty stable, as long as the machine is properly warmed up. If the flow is too fast grind finer -- the size of the grind is immaterial. Similarly one can experiment with slightly cooler or hotter brew temp. As long as a setting is repeatable and the change is predictable I don't think the absolute value is really needed. That said if you don't have a scale to weigh dose and output your results will be all over the place.