Í guess as long as it is in the filters outlet tube, in front of the diffuser/reactor the measurment is most accurate. because all co2 comming from the filter id fully dissolved in the water. But i think this is peanuts, it circulates in such a rate you wont measure much difference where ever you put it.. As long as you do not bubble on top of it.. But what is not dissolved obviously wont be measured.
One thing more important, i do not know if this has improved on the newer types of Milwaukee's, but the SMS122 as i'm using is very sensitive to electrical interference like stray voltage and especialy, the pumps magnetic field. Reports of this are to find all over the net, the buzzing relai..If that is the case you could see a the pH value on the meter constantly fluctuate with 0.1 or 0.2 units. The set threshold probably is factory set at 0.1, so if there is interference and reaches the desired set value, the relai will rapidly click, switching on and off, or even start to buzz like mad i duet with the solenoid. Caused by the fluctuation..
If you experience this than remove the probe as far away from the source causing it.
In my case i'd put it behind the heater in front of the reactor as far away from the pump. Also outside the cabinet, because there is to much elecytronics in the cabinet which could cause interference. Just to be sure it doesn't occure.. Mind if it does and you're not at home, it'll buzz for hours to come and might burn or wear out and trash the relai or solenoid..
Thank you for the hint. Very helpful for me when planning the layout inside the cabinet.