A basic consideration is that if your buddy's regulator or tank has a major failure, that you need to have enough air available for the two of you to safely end the dive.
A search on "rock bottom" will reveal many threads about that topic.
While not perfect, a reasonable approximation of the needed reserve of air in an aluminum 80 tank is about 100psi for each 10' of depth, but never less that 700 or so for starting the ascent.
Some people will use a slightly higher number, such as 100psi/10' + another 300 or 400 psi padding. A new diver, with higher air consumption rate should add this much or more.
For example, if you are at 80' depth, you should start your ascent when you reach 1100psi or thereabouts. Assuming the normal case where you don't have to share air with your buddy, that should give you plenty of air to do a nice slow ascent, safety stop, and still have a reasonable reserve for doing an exit through surf or breathing on your reg while boarding a boat.
Again, it's not a case of how much air one should have left on the boat. It's a case of always having enough air for both you and your buddy to safely end the dive while both breathing the air from your tank. Your buddy is carrying your reserve air you will use if your tank or reg fails, so obviously it is in your interest for him to start the ascent with the proper reserve.
Charlie Allen