When I am doing a 1.5 cubic I am using a capacity of 33K with the 9lbs of salt.
I would use the 15 grains in the programing, better to be a little on the high side than short.
I do not have on hand the link , but if you look up the Clack home page there should be a part that has different programing of the different controls and valves that are out there.. I have them downloaded, but not the site from which they came from.
Just my 2 cents worth.