Knowing the rpm at which the total advance is in would help as it is important in the grand scheme of things.   When performing a total advance reading, you essentially raise the rpms until the timing reading at the damper peaks out.  With stock distributor advance springs and with the vacuum advance line disconnected from the distributor and plugged, the engine will likely be over 4000 rpms before all the mechanical advance in the distrubutor is ‘in’ or taken up.  With the vacuum line disconnected, 36°-40° total is the value to shoot for.

With the vacuum line reconnected, the total timing will obviously be more once the engine rpms are brought up and 50° ignition timing is not to be unexpected as it’s now mechanical and vacuum advance working together.  If the distributor has been recurved by using lighter springs, then the amount of advance being supplemented by the vacuum should be reduced as it can now be too much advance at the lower rpms under a light load situation.  As I mentioned earlier, the rpms are a definite player.  And as Charlie mentions, without a load on the engine, the vacuum to the distributor will be intensified and thus give a higher no-load timing reading.  Under a load, the throttle blades are open more and because the port in the carburetor that supplies the vacuum to the distributor is further away from the throttle blades versus what the position is when simply reving the engine in neutral (no-load), the vacuum signal is diminished.  Engine vacuum as a whole is also reduced under a load and the ported vacuum signal also reduces along with this.  As the throttle blades continue to move away from this port, the vacuum signal eventually disappears thus allowing the engine to rely solely on the mechanical advance feature of the distributor.

Those familiar with this unit will see that the travel stop is in the "18L" slot - which is 36° crank degrees of total centrifugal advance. If you "power time" the engine as I usually do (raise the engine speed to full advance to set timing at 36°-38°) - when you let it return to idle, the initial will be way too low. if you do as the picture suggests (move to the "13L" slot) - the full centrifugal will be 26° - and the "power timing" procedure suggested above, will raise the resulting initial at idle to 10°-12°. This is all done with the vacuum disconnected - plugged at the carb. I like to change the springs too - so the advance curve is quicker - but that is another subject.

There are two types of advance pots around....this one with shims springs and a fiber tube stop.

And this one that adjusts with a 3/32 hex wrench.

They may be set to deliver as much as 25° advance at 15 inches Hg. If you are using a quicker mechanical advance curve in your distributor (as described above) - you have to turn them back to keep the engine from knocking at part throttle. Adjustment is basically trial and error - drive it/adjust it sort of thing. Hoosier suggested a good trick I use on mine - orfice the vacuum line to the dizzy so it doesn't change the pot position so fast - dampens things a bit.

http://www.highperformancepontiac.com/tech/0212hpp_spark_plug_advance/index.html

http://www.hotrodders.com/forum/distributor-tuning-theory-part-1-a-59033.html