I have noticed the plates in some distributors lie, it will say L12 or L10 or L6 which is suppose to be a range of 24 degrees and 20 degrees , which is suppose to be 12 degrees
when you setup the total timing, you expect say 34-36 total timing, (initial+ no vacuum advance) yet when put the timing light on the balancer and rev the motor you will see 40-54 degrees of timing. I spoke to an engine builder about this and he has seen it also on a lot of re-manufactured distributors he told me that he has had to take distributors apart reweld the timing plates then Cut them out to proper specs, or you can use a piece of vacuum hose over the stoppers to shorten the throw on the distributor timing.
I had to do this.
as for my distributor tune.. I had a wicked mid throttle engine ping/detonation, my builder installed a set of mr gasket 2500 rpm springs. in addition to using a piece of vacuum hose to limit max timing, i had to swap out the mr gasket springs for a set of OEM springs(heavy spring for over 2000rpms and the light spring for the instant off idle timing advance. It was a pain setting the play on the heavy spring so the heavy spring would not hold back the timing below 1500. This held back the distributor mid throttle so over 2500rpms i get my total 36 degrees. The rule is you want all mechanical timing in before 3000rpms for performance. in my case my engine would not tolerate this due to low compression due to open chamber heads and piston combination.
I also needed to cover up the lean ping with a 50cc accelerator pump and 35 shooter size, along with a 8.5 power valve, this was on a Holley DP 750CFM
i was able to tune the motor from 10HG" to 17 HG" with the changes i made so it made sense to change the power valve. Total HG at idle / 2 +.5 = for your power valve
i didn't have a 9HG power valve so i settled for a 8.5 instead of a 9.5HG valve. I remember i did try and up my Jet size from 70 to 72 but i had a hot start issue and at WOT i seemed to be ok, it was always a mid throttle issue with too much timing too soon causing me problems.
So distributor tuning may also involve holding back timing just enough to work.
later on i tuned my Vacuum advance, getting some timing back for cruise.
It is a lost art but what i found also was sometimes the book rules do not apply and you really need to listen to the engine's sound when you are driving, you can hear a slight miss or ping and know your distributor may need a change or you need more fuel in a certain rpm range.
anytime you move away from the stock configuration on a motor, you end up in la la land as far as what works with the parts combo you have.