The important thing to key on is the volume of the tank. In the US, the "volume" of the tank (66 cft, 80 cft, 108 cft etc) is based on the amount of "standard" air contained in the tank when filled to the rated pressure (including any "plus" rating). So, 500 psi in a 108 cft tank is more air than 500 psi in a 80 cft tank. So assuming you trained like most folks on an aluminum 80, your 108 will have a bunch more air than you are used to at the same pressure. The fact that you start at a lower pressure (2640 vs 3000) is irrelevant. Full is full. 500 psi in a big tank is more air than 500 psi in a small tank.
Secondly and as others have mentioned, the "500 psi rule" is a rough guideline to help you keep out of trouble. 500 psi in an aluminum 80 should be enough to get your and your buddy (air share) to the surface from the depth of any recreational dive, but don't dawdle! <grin>. However, I have some thoughts on the subject.
As others have alluded to, 500 psi is the "back on the boat" pressure, not the "lets start looking for the boat" pressure. That 500 psi belongs to your buddy - not to you. You and your buddy need to agree on a turn pressure based on what ever rules you are comfortable with (rule of thirds, 1/2 of the pressure above 500 psi, whatever). You and your buddy must agree on this and then plan the dive/dive to plan to get back on the boat with that 500 psi in the tank (assuming no emergency).
For normal dives you should plan to be back on the boat (shore) with 500 psi on the tank. Now, I'm not super rigid on this, and if you and your buddy are both on the anchor line doing your safety stops while breathing off your own tanks and you dip into that 500 psi while the gaggle on the ladder/current line clears that's ok by me. But if my students are farting around at 100 feet with 600 psi in the tank they are going to get a friendly reminder about dive planning.
Another thing to remember is that regardless of the volume of the tank, some regulators start to act funky if tank pressure gets too low. The typical IP for a regulator system is 140 psi above ambient. If you attempt to stay deep with low tank pressure breathing effort will start to increase rapidly. Again, this behaviour is independent of tank volume. A bigger tank will get to lower pressure slower, but once you get to low pressure, the regulator behaviour will be the same. As others have mentioned, SPGs are none to accurate at the low end of the range, so getting way into the red is not a good idea in any event.
Bottom line - do your dive planning the way you normally do. You and your buddy must agree on a maximum dive depth, maximum dive time, and minimum tank pressure (turn pressure). If you have a bigger tank, your buddy may reach turn pressure before you. That does not matter, someone has to be first.