Concern for a Pin-Lock Keg Leak

I just recently fininished my kegerator build with plans to post pictures later. In efforts of avoiding a leak for a leak in the gas line by pressurizing the line and turning off the tank. Both manifolds held pressure when open with no issues. I pressurized the pin lock keg and let it sit for a few hours and that also held pressure without issue. I have replaced both post rubber rings along with the lid rubber ring using food-grade keglube. My CO2 tank was sitting at 800PSI when fully pressurized (It's an old tank I had rehydrotested and filled so i'm assuming it's full) When I connected all the lines and started carbing my first homebrew beer and dropped the fridge from 20 C 5.5 C (~75F->42F) the tank pressure dropped from 800PSI to 550/500 PSI.
No soapy water has shown any bubbles but i'm still concerned. How accurate are the tank pressure gauges (It's a taprite dual body triple gauge regulator) as this one is at the lower limit of green. I'm assuming dropping the temperature and starting to carbonate a keg will drop the pressure of a 20 pound tank however I'm looking for more experienced individuals for reassurance.

In all likelihood you're just seeing the drop in pressure associated with cooling the tank. An easy way to check is to remove it from the cold environment and let it warm back up to room temperature.

As the other poster said, the cold fridge probably accounts for your drop in pressure. More than likely you didnt lose any CO2, it happens to me all the time.

Billy