I doubt if it needs more than 12 degrees in addition to the mechanical curve. The compression ratio on my similar 455 is a little higher and the older version of the OF cam. I've varied the VA timing from 10 to 15 via ported vacuum and it doesn't much care.
"ported vacuum advance adds 12 at cruise( need to check this as well) I've messed with the timing so often always trying to find the sweet spot for cruise and also for the track."
I LOVE It! I'm sure you know that it's not possible to use ported vacuum to the advance and get good idle quality plus 18 mpg's w/o overdrive. You've got to be careful putting info like that up here, now we're going to have to put a few Forum members on suicide watch!
Seriously, I'm actually glad you did and not me, because every time I mention that it starts a chain of PM's from my "fan club" telling others how I don't know jack squat about tuning because I'm not using manifold vacuum to the advance......LOL.....
__________________
If you can read this, thank a teacher. If you can read this in English, thank a Veteran!
https://cliffshighperformance.com/
73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
|