This issue comes up here on a regular basis (like, monthly). Stow the OCD and ignore the gauges is the usual advice. I have a black tank flusher and can look directly into the tank. Every 4th dump or so, I dump and flush, then refill with the flusher to 1/2 full, then dump again and keep doing that until the flush water comes out clear. I look into the black tank through the toilet and I see the bottom of the tank. The gauge lights read 1/2 full. My gray tank gauges have never indicated less than 3/4. I've done Geo and Simple Green and it makes no difference.
SO... stow the OCD and ignore the gauges.