The Zimmerman calculation works like this:
hf = 30 – (Today’s average humidity)
tf = ((Today’s Average Temp)-70)*4
rf = -((Precip Today)*200)
Scale = 100+(hf+tf+rf)
The scale is then bound to 0 to 200
If you notice the results are skewed too much in one direction or another they will often remain skewed within the same season ie summer as overall weather patterns are fairly consistent. To correct for this without manually running your program all the time I would suggest finding how much water you believe you are needing or not needing and adjusting your programs watering time by that percentage. For example you have a watering time of 10min programmed but it keeps knocking it down to 5min and you really need 8min. Adjust your pre-scaled time by +30% (100*(8-5)/10) and you will now have something the weather calculation will work with, just know that you may need to watch it for a few cycles, and if you have watering time restrictions you could run over your allotted time, though this could happen with the base calculation.
Also, his calculation works well in most areas away from large water sources ie ocean/lake, so prior to messing with the times make sure it actually needs more water. It’s easy to check by stepping on your grass, if it springs up within a couple seconds it does not need more water. Grass is ideally kept right on the edge of its drought conservation behavior, the plant will hold water near the roots and use surprisingly little water while remaining green. I used to work for a commercial irrigation company and was surprised by how much water people would put on their grass with little to no benefit, or worse causing fungus growth and killing large patches of grass. Remember, grass will yellow if it is over watered as well, so color is not the best indicator. For shrubs and other plants check with a knowledgeable person at a nursery for best watering practices, or ask how to tell if it is drought stressed.
It appears as though ET is in the works, the script used for weather adjustments has the calculations present, it just isn’t implemented at this time, so given a bit of time Ray and Samer may have the solution soon. (ET is evapotranspiration, a complex calculation for actual plant water loss based on several environmental factors, including wind, temp, humidity, and solar radiation to name a few.)