Do you know? Thermocoupler

[walkerj001]

Hello I am new to the forums and got pointed to this forum for assistance. This may be a basic question , but I am still learning and wanted to be sure I am doing this correctly. What I am doing is installing a thermocouple into my chimney pipe so I can get a digital readout on a PID controller from inside the house. The question I have is what would be the best location to install this thermocouple? I have seen where people stated 18" from top of woodstove, but that was with woodstoves that had chimney pipe coming out from the top of the woodstove and not the back. I have attached some pictures for reference. Also I know that thermocouple should be before the barometric draft regulator.

Unit is a US Stove 1557m

Thanks in advanced for your assistance !
Greatly appreciated