Jah Jah, there is some nonsense going around that a regulator will "bottom" when the tank has zero psi, and that there will be no indication that you are running out of air before it bottoms. There is no truth in this. The regulator will start to bottom when the tank has about 120 psi gauge. This is easy to recognize; simply, the regulator will seem hard to breath. After you check the valve knob and the gauge and confirm this situation, begin a slow swimming ascent. As you ascend, the water pressure will diminish and the gauge pressure will increase causing your air supply to increase keeping pace with your swimming ascent. However, you may have to rise at a speed of 60 fpm vice 30 fpm. Don't be concerned about this unless you are already in deco status, not likely in most situations which you will encounter as a newbie diver. I'm telling you this because the "out of air" situation is almost always due to diver error or a stuck gauge and not regulator malfunction--and that you can bale safely. In the old days, a swimming ascent was not uncommon. I have tested a MK20/G250 in a "no air" situation and it behaves the same as my double hose regulator which was manufactured in 1965. I specifically chose the Scubapro reg because they originated the "large piston" air flow story which lead to the myth.
Gauge pressure is also known as "differential pressure" and even though your SPG may read zero or near zero, some air will remain in the tank.