here is my take...
Dwell is an important measurement in the timing circuit of your engine. The goal is to get as much dwell (closed points) between spark plug discharges as possible. The longer the dwell time, the more charge built up in the coil and the stronger the subsequent spark. Unfortunately, with mechanical points systems, there is a physical limitation to how long the points can "dwell" closed and this is where the "gap" specification comes in. The ideal would be to have an infinitely small gap so when one of the points cam lobes bumped the moving point open, it would be an infinitesimally small opening, just long enough to collapse the magnetic field in the coil. And BAM, you get a bigger spark... or at least the biggest available spark...
In real life, that is an impossible goal with mechanical points. First, it is impossible to machine the parts with that precision. Even if you could machine to that precision, ANY wear or thermal expansion would destroy spacial the relationship of the parts. And this kind of wear happens rather quickly during engine use. Add to that the nasty tendency of all that inductive electrical load to arc across a small gap and you are left with needing to set the points with the equivalent of a grand canyon sized gap. Any sized gap isn't a good thing with respect to hot sparks, just a physical necessity.
That is why some high end mechanical distributors are fitted with dual points. The two sets of points are installed slightly out of phase so when one set opens with it's necessary big gap, the other set is still closed. When the second set finally opens with it's also large gap, the first set of points is just closing. Thus, this is a mechanical way to achieve the 'infinitely small' gap, longest possible dwell, the highest possible charge in the coil, and the hotest possible spark.
Electronic, points-less ignitions achieve the goal of infinitely small gap, maximum dwell by replacing the crude mechanical points with a transistor switch. Thus the "gap" is the size of an electron and opens and closes at the speed of electron flow. Add to that, unrestricted engine speed capability, since there are no mechanical parts to accelerate open and closed, ZERO maintenance, relatively low cost. So ask yourself, why would one even give a points system a second thought?
One last shot (thought)... I used to check the dwell all the time, until i stumbled upon a book, published back in the 40's, on achieving good intake vacuum under driving conditions. my first car, 68 Plymouth Baracuda, had a guage in the dash called a fuel efficiency gauge, and it had a few numbers on the guage face, 5, 10, 15, 20, and 25. In those years, i did not know that this was a simple vacuum guage so you could adjust your foot throttle to increase the vacuum to achieve better fuel economy. Simple carbureator function...
interestingly enough...
the same book had a section on adjusting the timing for better vacuum. what i discovered was an "art" of timing my engines. once I get my points close, I can move the timing a bit to a place where the vacuum in the intake is maximized during the four stroke proccess. in order to properly see this, you will need to learn to read the "carnot" or "engine cycle" maps. if you ever get a chance to talk to a race car engineer, ask them about the function of the "cam" verses "timing" and "spark."
later in life, during my military years, we opened the hood, and watched the vibrations of the edge of the hood as an indicator of the smoothness of the engine cycle map. all of this "art" work pays off if you change points every 3k miles, and get used to "seeing" the cycle of the engine work correctly.
another point... my dizzy, or distrubuter, has an external thumb screw/adjustment for advancing and retarding the timing for different octane fuel types, and also for elevation issues with fuel. i have found this a useful tool in adjusting the dwell with my oscilloscope, it's really more than the standard shop guy wants to do to his or her engine timing, but once you get "into" your engine, then these tools are like wide open windows from which to spend quality time with your fj40.
sweet, joyful, greasy, late into the night, fj time. just how sweet can i get this engine to run???
. The dwell reading is at 28 on my combo RPM/Dwell/ Volt meter. I am out of OEM spec, but it is runninng well. Should I be concerned? Will it cause any damage? I did have to advance the timing to increase vacuum enough make it run without sputtering. RPM's are 650, steady vacuum is 18, and idle mixture screw is about four and a half turns out. Thanks for any info.
Brendon
. It is quite an education for a 
mechanic like me. Now I have to go back and reread everything and try to digest it all again....