Thanks guys.

Still playing and hopefully getting closer but I still have one core giving rounding errors half way through HyperPi (not alwats the same core!). I go back and change some GTLs and then the other core errors.

I have given up on the science and the .63x and .67x multipliers as I don't know which GTL this applies to in my bios. I tend to agree that at limits of hardware then the exact formula does not apply and a lot of trial and error is needed.

So I need a more pragmatic approach, I like the suggestion of isolating a core at a time. I was running HyperPi on all 4 cores but now wish to run 2 instances of it - on one core each - so that I can isolate a die at a time. The first runs will be on cores 0 & 1 = die 0 = GTLs 0 & 2 and then I'll work on die 1. I have seen in task manager using 'set affinity' how to select CPU but have do I select which core each instance (process) of HyperPi to use ?.

Just to test the positive/negative part of the equation I ran GTLs at +50 and had an average core temp of 58, at GTLs of -50 it was an average of 6 degrees lower so neagatives win for me !!.