The hardest thing is recognizing when a beer is really carbonated vs. when a beer has simply been blown through a serving line under intense pressure. They both have a foamy head but only one is a result of CO2 coming out of solution that was originally dissolved in the beer.
I can make a few more true statements that might also help. There are a few things that increase the speed at which CO2 will dissolve in the beer (the carbonation process).
1. Higher pressure applied to the headspace. We know that based on the charts, a higher pressure will ultimately end up at a carbonation level well higher. Even if you target is 2 volumes, setting the pressure so that it would ultimately reach 4 volumes will get you to 2 volumes quicker. BUT!!!! you'll never really know when to stop and purge. It's a guess.
2. More CO2 to beer surface area. In a typical corny, the contact between beer and co2 is about 28 square inches (the area of the surface of the beer). That's a pretty small area give 5 gallons of beer. The tricks to increasing surface area is physical agitation and gas diffusion. When you agitate the keg, it makes the headspace of co2 turn into a million bubbles immersed in the beer presents a much larger surface area than 28sqin. You can also achieve a similar effect by putting a stainless stone on the incoming gas and having it immersed in the beer. Using either of these surface area techniques with the pressure from the chart is the ONLY way to be 100% sure that you are not overcarbing in the process.
Of course, you can do a combination of these to get there faster but the odds of overdoing are increased.
Just like in my first post, the only time I'd agree with speeding carbonation is if the beer has already been properly aged already. If not, the tried and true set and forget method accomplishes a predictable carb level and aging in one process.
I also don't want people to think that just because they increase pressure and agitate the keg that carbonation is absolutely instantaneous. Just for discussion's sake, let's say you have a 35F keg of beer and you raise the pressure to 30psi. Given the chart, with about 2-3 weeks wait, the beer will achieve 4.5 volumes of co2. Maybe you really want 2 volumes but you're trying to get it done faster. Is it 2 days? 3 days? I'm not sure, but it's definitely not 2 weeks. Now, same example with temp/pressure but let's say you shake the crap out of the keg at the same time for 5 minutes. How many volumes are you at immediately after? Who knows, it could be 2 volumes.
If you're into being exactly to style and want reasonable accuracy on carb levels, stick with chart and wait. If you really want to hurry but still be reasonably accurate, use the speed methods and then get a bleeder valve with a pressure gauge on it. How's what work?
If you have a keg sitting around that is in some state of fixed carbonation but you don't know what that is, you can put a pressure gauge on it to find out. The caveat is that it has to be a fixed carb level where the CO2 dissolved in the beer is at equilibrium with the pressure in the headspace. If that is met, you can read the head pressure, measure the beer temp and find the volumes on the chart.