Very interesting.
Ok, simple question - you turned off the pump and the SWG during the OCLT, right?
To answer your previous question - a 4 month old cell with clean plates should generate the as-spec'd amount of chlorine. It would be a rare circumstance that the cell is somehow underperforming. Unfortunately the only way to check a cell is to bring it to a pool store that has the equipment to check Pentair cells. There is a special wand needed (yes, it's called an IntelliWand) to connect a laptop to the cell wirelessly (it's uses a proprietary RF signal) in order to readout volts, amps, cell temp, etc. It's really one of the more annoying design features of Pentair SWGs. An authorized service center can do those checks.
As long as you are seeing little gas bubbles at your returns from the cell running, you can assume it's generating the rated amount of chlorine. You can use a clean water bottle to collect a water sample directly from the return and the FC should measure very high. Hard to say how much it will be but it should be a lot higher than the pool water.