I have started dives in calm water, but surfaced with choppy water, with 3-5 foot waves, with about 500psi. I go to my snorkel, and wait for the boat to pick me up. The last 500psi is for getting on the boat.
If you rent a tank, you do not want to run it empty. I will cost you alot returning an empty tank.
The last 500 psi if for emergencies and it's contingency gas. If you have to use it, use it! I've surfaced with less that 500 psi a few times and boarded a boat with less more times than that because I've needed it for some reason, that's why it's there!
R