Here is the general idea. Minimum FC level is a number that you never want to drop below, ever. Target is a suggestion because most SWG pools will lose less than 2 ppm from the time the SWG shuts off to the time it starts up again. Any number above target is fine, whatever number is needed to make sure that the FC doesn't drop below minimum. It is safe to swim with FC up to shock level for your CYA.
In your case with CYA of 75, we round to 80, because the CYA test is logarithmic, so halfway between 70 and 80 is not 75. With CYA of 80 your minimum FC level is 4 ppm. Which means that you do not want your FC to be below 4 when the SWG starts up for the first time of the day. If 6 covers that then good. If not, then target 7 or 8 or whatever be plumber is necessary to make sure that FC is 4 when the SWG starts for the day. It is safe to swim with FC levels up to 31 ppm. So, you could safely keep you FC at 8 or 9 if you wanted to or needed to.