Christy, IMO get the tank refilled once the pressure starts falling. Watching the pressure drop for another week or two doesn't save any refill cost by using whats left (time/dollars.)
The reason I'm asking about regulator testing is because there seems to be a problem with a regulator used by a canreefer. He is using a 5lb tank and it has lasted a week until the pressure started to drop on the high pressure guage. Initially the tank was at approx 750psi. I've heard rumors that the previous owner had the co2 tank suddenly empty out thru the regulator. Could it be a pressure relief valve problem or just a plain screwed regulator
I thought that they would either leak or not leak. This one seems to be unpredictable.