You can use the measurement of PAR right where the plants you are trying to grow are located. That will be a range of numbers - high near the waterline, lower near the substrate. I think, for us, a better use of a measurement of PAR is to substitute for "watts per gallon" as a measure of how much light we have. That requires measuring the PAR at the substrate, in the middle of the tank, and using that number as a measure of how much light the tank has. That is how I use it.
That number should be around 20-35 if you want a low light tank, around 40-50 or a little higher, for medium light, and around 60-100 for high light.
The units of PAR are micromols of photons per square meter per second, but most of use just say 20 PAR, not 20 micromols of photons per square meter per second.