PiezoDN negative is a very stable negative making process. It is not sensitive to minute changes in chemistry as is using color inks to print negs. PiezoDN has a very wide latitude because of the amount of overlapping monochromatic inks none of which are peculiarly more or less sensitive to UV as are the different color inks that make up a color ink neg. If your experience has been in the past that you needed to recalibrate to the least minor change in your chemistry or environment, you should find that you can go much wider in swing with PiezoDN negs without visibly throwing off the print.
Ferric Oxalate, at least in the data we have recorded is one of the most sensitive of the solutions you are using. So, we mix FO every 60 days. It is SOOOOOOO CHEAP to buy that you should not even consider whether you should try and extend its life. We are hitting dMax of 1.41 to 1.51 regularly depending upon paper. Getting such good black, and knowing it is partially dependent upon the FO - we don’t worry about discarding FO in favor of fresh. We removed the FO aging factor from our darkroom.
Having said that, humidity is probably the single most important factor in standardizing. Calibration therefore, is to first control your standard environment - otherwise you may be chasing calibration all the time and making very few prints.
Our practice is to re-calibrate when we mix new FO. On any given day or even late in the night - I can go into our darkroom and make predictable prints. When we re-calibrate we coat two small sheets each of our 4 standard papers. One for the target and one for the confirmation print. We can do it all in 4 hours or so and our darkroom is back to harmony. We do not need to chase minute environmental changes because we more or less control our humidity and temperature.