" he had a digatal salt meter that showed the salt level to be 3400ppm, my test showed 3600ppm, the ic40 showed 2300ppm."
With my math above will show this being correct
" Btw the water was 64 deg" - will make the cell read lower then norm.
"all summer (78 / 87) the ic40 has read from 500 to 1000ppm less than the Taylor test. There were a couple days when it was nearly correct but then the next day it went back to +-1000ppm low."
the cell it self will have a 200 to 500 ppm deff. Your taylor will not even be correct. That why you are seeing so many readings that are not the same.
If you take your pool water to three stores on the same day not one reading will be the same when it comes to salt. it will always have a + or - 500ppm.
some days it was almost correct? tell me its right. nothing wrong. What if your SI? When was it last checked?