I have always started at 12 deg.

Sounds like you have 16* mechanical advance + 8* = 24* total advance.
Give it 4* more initial and see what it does.(12*)
Check it to see if it 'exceeds' 36* (above 2000RPM.) If so how much?
The light advance springs let it advance.... quicker(at a lower RPM)
Sometimes....stock heads need even more than 36* total advance!
They are pretty inefficient....need a LOTTA lead at upper RPM's
36* is a good average.....
Lotsa factors are involved too......type of tranny.....final gear ratio...weight of vehicle....
type/grade of FUEL (big one!)...condition of starter....condition of battery.
Be sure and 're-tune' your carb.....(should be able to lower curb idle...decrease size of transition slot.) There should be an improved  "snap" in rev-ability at idle.
6s6

Just a 'wild-a$$ guess' butt   with A/T and 2.80 gears you MAY....get a little rattle at low speed and high gear acceleration.
One of those times you'll hafta play-with-it and tune it with different springs....carb adjustment....etc.
6sal6

Does the timing continue to increase as RPM increases?  At 3,000RPM do you still only have 24 degrees, or is it now 30?

There are two things that can be changed inside the distributor with regard to timing: springs and the breaker plate.  The breaker plate will have two slots, something like 10 and 13.  You get double the number stamped into the plate, so 10 is 20 and 13 is 26.  The arm only rides in one slot.  One way to get more advance without having to run a ton of initial advance is to flip the plate 180 degrees and put the arm in the slot with the higher number (if it isn't already).  You do have to pull the distributor to do this.

The other option is to change the springs.  The springs determine how fast the advance comes in.  Usually you can just go to the lightest springs in the spring kit.  The idea is to get all the advance in as quick as you can.  All mine is in by 3,000RPM and I run 38 degrees total advance (12 initial and 26 mechanical).  That setup works well for a stockish 289.  The more high performance the engine is the less lead it should need.  A performance engine is going to be maxed around 32 degrees.  The less efficient the chambers are, the lower the compression, etc. the more lead you want. 

If your compression is below 9:1 and the cam isn't lumpy I wouldn't be afraid to add more timing and see if performance improves.  I've found low performance engines sometime need as much as 42 degrees to really wake up. I run 38 on my 289, but plan to try more if I ever get to the track and they aren't closed (been trying to go with my boy for three years now and the two years before last they were closed when we went despite saying they were open on their website and voicemail).   I've always track tuned my vehicles.  You can only tell so much by the butt dyno.  Once I get it to hook consistently I can mess with timing and mixture and see if I can pick up some power.  A 0.10 reduction in ET is equivalent to 10HP.