Ok. Yes the screw adjustment 'starts' when the vacuum can kicks in. However: It does, in a sense, ALSO limit the amount of timing added, because the can adds advance over a range of vacuum. Meaning, if your can is adjusted to start adding advance at say 15", the spring does not slam the diaphragm into full effect at that point. As the vacuum increases, the amount of advance will increase, to a point.
I encourage everyone--hook a vacuum gauge up to manifold vacuum, to where you can see it while driving, and go drive and note the readings at different speeds/throttle conditions.
Then go home and hook up a vacuum tester like a mity-vac to the distributor can, with the cap off so you can watch the advance plate's movement. You'll see exactly how and at what vacuum reading the can responds as vacuum increases.
Typically, they'll continue moving the plate over about a 10" vacuum range(depending on the can used) so for example if you set the can to activate at 15", and the most your engine makes is say 18", it won't add as much timing as it would if you set it to activate at 10". So if you know how your engine acts you can control the 'how much' by controlling the 'when'. (Of course, seeing the actual # of degrees added must be done with your timing light).
On mine, out of the box the vacuum advanced my timing to over 60° because it started too soon. I adjusted it so that my overall total is right about 50° which the engine seems happy with. Whew! Long-winded sorry but I hope that helps.
*FYI cans can be modified to provide a greater or shorter duration of diaphragm operation as well