What is the shape and average depth of your pool?
I would call it symmetrical (lengthwise) kidney-shaped where it's narrowed in the middle and wider on the ends. Close to ovular though. Avg. depth is 6'. Here is the only pic I have on hand at the moment.
Did you ever truly finish your SLAM? (i.e., passed an OCLT)? That could explain your chlorine disappearing.
After passing, you should be able to dose chlorine with pump running and check your levels after 30 minutes. You should be able to zone in on your capacity (gallons) almost exactly. Especially at night with no Sun on Pool and nobody swimming. PoolMath is VERY accurate that way
Yes, have checked the CC and OCLT levels at least a few times over the past couple weeks. Overnight it's reading as it should. It's the initial level after adding chlorine I'm having trouble with. Perhaps my evening tests are reading less than actual for some reason. I've started a spreadsheet. Going to track meticulously until something makes sense I guess. I'm only able to test mornings and evenings before and after work. Hopefully will get a better indication of how much FC I lose during the day as well.