I always try to get the probe on the flame front. Every set up is a bit different but my rule of thumb is 100mm from piston skirt. That doesn't ensure you are on the flame front, but is ball park. I like the flame front as it reacts quicker and a good running motor can see over 1300*.
The further out you get, it has been my experience that the temp run cooler. Also, it has been my experience that the PTO cyl works harder (makes more heat), triples and twins.
So, for a compromise, 100mm from PTO skirt is where I would start.
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