If your softener has been using less than 15 lbs of softener salt per cuft of resin per regeneration, you get the maximum K of capacity of 30K per regeneration. It is a very rare softener that is programmed that way.
Most are programmed to use much less salt than 15 lbs/cuft of resin so.. you don't get the maximum K of capacity meaning not all the resin is regenerated, just the amount of capacity you use based on the hardness and number of people using water on a daily basis. IF you have used more capacity than the softener has been programmed for, yuo get hard water through the softener.
Example... you have a 1 cuft (32K as they are called although you don't get more than 30K/cuft) and it is programmed for 6 lbs of salt per cuft which regenerates 20K, leaving 10K of the original K of capacity still in the resin/softener.
So some day you use more than 20K of capacity by say 3K. Only 20K is regenerated the next regen, leaving 7K.
Now do that overuse a number of times (like your drip irrigation) and you use up the 'extra' K of new resin capacity and then your 6 lbs can't regenerate the 20K the softener uses between regenerations based on metered gallons, and you get hard water through the softener and start looking for why that is.... and find nothing wrong with the softener's operation.
To cure that situation you need to change the salt dose to 15lbs and do 2 manual regenerations one right after the other with no water use during or between the two so you regenerate all the resin back to 30K/cuft. Then change the salt dose back to the 6lbs or whatever is was originally and don't run irrigation water through the softener anymore.
You use potassium chloride and should increase the salt dose by 12-30% to equal the capacity that that much less sodium chloride would require for the same K of capacity. Or, my suggestion is to switch to salt and save some money over buying expensive potassium chloride.
OK, I'm still trying to figure this out.
I read the manual again and the only salt adjustable setting is the salt efficiency setting and it's either on or off. Default is on which says "at least 4k grains of hardness are removed per pound of salt.
My softener is a 39k grain system so wouldn't each regeneration regenerate the whole 39k grains of hardness and use about 10 pounds of salt ?
I understand your math about using 6 pounds of salt to regenerate and using more water than usual and the water gets hard.
Besides the salt efficiency setting on or off I don't see any way to adjust how much salt is used to regenerate.
My raw water hardness is 12gpg so I set the hardness to 12 (no iron in the water) then add 30% for potassium so I set hardness to 15.5.
Then set the salt level to whatever the level is in the tank from 1-8. Current level 5 so I set the salt lever to 5.
And you said not all the resin is regenerated, just the amount of capacity used based on hardness and daily usage.
I thought a regeneration fully cleans all the resin and makes all of them new again ready to make the water soft.
You said to fix it change the salt dosage to 15lbs and do two regenerations then reset to 6 lbs.
Since I have no salt usage adjustment (only salt efficiency on/off) then wouldn't a regeneration take about 10lbs of salt no matter what ?
Then by how many gallons run through the softener the computer knows when to regenerate and clean the entire 1 cubic foot of resin that's in the tank then the process starts all over again ?
I'm sorry if I still am not understanding how a softener works.