Single wall temp measuring - probe vs surface

There's a large discrepancy on what internal single wall flue temps are. Recently we've seen postings with a a multiplier for surface temps going from a third higher to 100% higher. I've usually struck in between at 50% higher. And then we have the variations of thermostats that can be 100F off. Which is correct?

Does anyone have instrumentation on single wall where they have measured with a thermocouple probe the difference between a Condar or Rutland pipe thermometer on single wall pipe and the actual flue gas temp?

Correct. 50% higher than surface is what I recall and have used, but I was hoping to get more data points from folks that have since installed digital probes. We have double-wall connector so I can't verify on our stove.

Thanks. Temps will vary with the setup I think. My expectation is that there will be a different flue gas temp reading with systems that are straight up, than those that go up, take a 90 turn to an outside tee. Could be wrong though, but it would be interesting to know.