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