Vacuum is usually measured in inches of mercury, from standard air pressure down to absolute vacuum. Kind of like a backward barometer. Standard air pressure of 14.7psi is usually measured as zero on a pressure gauge. Any pressure above 0 is that number plus 14.7. Both can be measured and stated in a multitude of units but these are usually the units we use when dealing with our car's pressure and vacuum.
Vacuum in the carb and intake manifold is created by the evacuation of the downward movement of the piston, with the intake valve open, and the throttle plates closed. Once the throttle plate/plates are opened the carb and intake valve ore open to atmosphere and the vacuum is no longer there. The vacuum advance is operated by vacuum on one side of the advance can diaphragm and atmospheric pressure on the opposite side causing the can's arm to advance the timing in the distributor. In an HEI it rotates the pick up coil, thus advancing the timing. With the vacuum advance can hooked up to the intake manifold, with a vacuum present, the timing advances but when the throttle is opened the vacuum signal goes to zero and the spring in the advance can returns the advance to zero.
As Bob S says, the advance you are concerned with at full throttle is the total, initial and mechanical. The vacuum advance is not considered in total timing. The mechanical advance is dependent on rpm and doesn't vary with load. The timing is advanced so burning the fuel air charge results in maximum cylinder pressure when the piston is in the most effective position. The faster the engine rotates the earlier the fuel charge has to be ignited in order to have max cylinder pressure in the correct piston location. Mechanical advance starts off idle and is full in at somewhere about 3000 rpm. Optimum total timing varies with each vehicle depending on the camshaft, static and dynamic compression ratio, rear gearing, combustion chamber design, etc. Most Pontiac engines run well with 12-16 initial and 18-24 mechanical but a simple cam swap could change what the car needs. Timing affects power, drivability and cooling, too much may cause engine damaging detonation, to little could cause low power and overheating. We want to set the timing so the car will start easily, idle smoothly and give good throttle response, once you set your total timing then tune your vacuum advance but do not adjust the total to vary the vacuum. Set total by driving the car and adjusting the timing up a degree of two at a time until you get a ping with a good pull in second gear, then back off two degrees. Usually best power is had just below detonation. If, once you have your max total set, you have too much initial to start the engine easily you will have to adjust your mechanical advance system to get the total with less initial.
The fuel air mixture is leaner at idle and cruise than it is pulling torque at wide open throttle, it may also be defused with exhaust. My engine idles at an air fuel ratio of 14.5:1 and has a richer mixture of 12:1 when pulling with the pedal to the floor. The leaner mixture at idle takes longer to burn than the richer mixture. To keep the fuel charge providing max cylinder pressure at the appropriate time vacuum advance is used to fire the charge earlier. As the intake manifold has vacuum at idle and stepped vacuum source does not, the manifold source is usually used, although not always. Some of the smog era cars had the idle timing retarded quite a bit in order to burn the charge as it went out the exhaust port to aid with burning contaminants along with the air injection reaction system. The problem with a late burning charge is it transfers heat to the coolant around the exhaust port. Adding 10 to 15 degrees of vacuum advance at idle and cruise will improve fuel economy and aid in engine cooling.
Every engine is different of course and what works for one wont necessarily work with the next. For mostly stock V8 an initial setting of about 12 degrees, a stock mechanical advance and the vacuum can connected to manifold should give satisfactory results. Some engines with a larger camshaft may benefit from less mechanical and more initial. My engine has the highest power and torque at 28 degrees total but runs smoother and cooler with 18 degrees initial and 5 degrees vacuum advance. In order to obtain those settings I had to modify the mechanical and vacuum advance systems. I got the info while tuning on a dyno,, before that I just listened for detonation and adjusted the timing accordingly then connected the vacuum advance to manifold. I don't think I can notice the difference while driving. I did notice the cooler idle though, it heated up when I had a lower idle advance.