Where to measure temperature in a 8" J-tube with start chamber?

I'm building a 8"J-tube with a half barrel stratification chamber.
I have 2 pyrometers (thermometers for up to 500 C).
Where would you measure? The riser is too hot, so anywhere after that would be an option.
Top of the bell?
Exit of the chimney?
Inside the mass?

I think I would go with top of the strat chamber in the beginning and then chimney temperature (where the bell goes into the chimney). What do you think?

Hi Ben;
One for sure in the exit chimney and another at the top of the strat chamber, at the farthest point from the core

Hey Tom. Thanks for the quick reply. As the chimney will be at the far end, far away from the core, chimney and far end of the strat chamber would be very close together. Would the top of the strat chamber, close to the core and chimney make sense?

To me, the most important place is the exit chimney.
At home, that is all I look at; really, it's the only one that really matters.
But other locations around the bell or riser are pretty neat to see, too.