6 pounds of salt per cu. ft. or total? At 6 pounds per cu. ft., 9 pounds total, program the capacity to 30K.
In general, the hardness is heald if the capacity is exceeded, it is usually not dumped. Check the raw and report back
As to the back to back regens at high salt, not sure why, if you understand the regeneration frequencies, reserves, capacities, etc... I have been doing heavy commmercial softening where we run the capacity to the maximum efficiency and we have never done the double regen with 15 pounds per. I am sure it wouldnt hurt, but it is a huge waste of salt. Residential systems usually are run at an 80+% efficiency, so every regen is done a little early so the capacity is always maintained a little higher than what a calculator would show. I would recommend adding a couple gallons of water to the salt tank prior to regenerating after you have done your testing and possibly reprogramming the valve for your water conditions and actual capacity. Each gallon of water dissolves approximately 3 pounds of salt, so 2 gallons would add an additional 6 pounds of salt usage to the system.
10% SF is taken from the available capacity.
Hope this helps.