1968 340 timing

Your distributor does at at least 4 different things.
1) Power-Timing
2) Idle-timing
3) PartThrottle, and
4) Cruise timing.

Of these,and IMO, the idle-timing has to be set first, to establish a starting point for the Power-Timing. Then, we already know that the end-point is gonna be at around 34/36. So you have to modify your D,in some way,to connect those two points. Then,you have to figure out between idle and WOT, what rate of advance the engine will tolerate without detonating .
Now you have established just two timings. There are, so far, just two times when the Timing is right, namely idle, and WOT. AT ALL other throttle settings, the timing is guaranteed to be wrong. Not an error on my part; AT ALL other throttle settings, the timing is guaranteed to be wrong.
This is where your V-can comes in. You the tuner ,have to figure out how much more, and sooner, that your engine wants PT(PartThrottle) timing . You can never hit all the PT points that your engine will call for, using the Factory-TYPE Distributor, So you just hit as many as you can, And IMO one of the most important of those, for me at least, is the Cruise Timing.

And I am always willing to modify or give up Power-Timing, or Idle-timing, or rate of advance;
to get the PT and Cruise timing that my engine wants;
Because ...... my engine spends most of it's life NOT AT WOT. lol.