The 'constriction' is the amount of reduction in inside diameter at the choke. To figure this out you'll need to know what the bore diameter is of the gun you're using. This used to be a standard, but now manufacturers have changed that. You'll need to do some research or get the right tool to figure out the bore of your gun. And to throw another wrench in the gears, the contriction chart is a rule of thumb, the actual choke is determined by 'patterning' your gun at 40 yards. You can find this information online as well.
There is a lot to this, so I tried to keep the answer relatively short. Again, you can find a lot of good info online.
Hey bwendholt...you forgot the exra full choke (turkey choke). I have one for my Remington 870SP Turkey shotgun. I don't know what the bore is though. At least I would think its has a little more restriction than the full.
You bring up a good point. I don't know how "extra full" chokes work since a 0.040 restriction is considered to be the maximun affective amount. They say anything more than that amount does not positively affect the pattern size. Perhaps extra full chokes have a similar overall restriction, but utilize a more gradual constriction angle which would explain why many of them protrude past the end of the barrel. Would be interesting to know more???