You did great.
Your decisions, considering how the DM handled it, were well made in my opinion.
Similar happened to me in Borneo. Just assumed the tank was full, as we didn't have much time between on the boat and on the site. It was only a couple of minutes from the dock, and I allowed myself to get rushed, so I didn't check.
We were going deep to photograph Coleman Shrimp. I rolled off, got to about 35 feet, and confirmed my readings, to realize that I had only 1700 psi. I waved "bye bye" to them and climbed back aboard. The DM helped me swap tanks, then I checked PSI, took a couple of practice drags on the reg to ensure it was open and rolled off again.
What I didn't know was the DM for some reason charged the tank, then turned off the air again (?!?!?). I was only 5-10 feet down so I did a CESA and opened my valve at the surface my self.
The next attempt, my ears just wouldn't clear. I guess that I wasn't meant to see them that day. I sat on the boat grumbling, to later only have everyone else raz me for not doing my own checks, and never assume that the DM's always got it right (these guys were generally very good though, I gotta say that).
As my friend got back aboard, he noted that he screwed up too. He put his DSLR into the housing, put it all together with the twin strobes on extended arms, but forgot to take off the camera's lense cap first.
Well, he got to lug that around for a whole dive, missing some very good shots too.
Bottom line of all of these stories?
NEVER, NEVER get rushed enough to miss doing your own checks, no matter how much you believe someone else is reliable. It's your final responsibility.