Idle Rpm High in Park, Low in Drive

Optimal idle advance.
The highest idle rpm, smoothest idle, highest vacuum with least amount of fuel used.
Two more quotes.
[1] Same H book." The more vac an engine pulls at idle, the more rarefied the charge is as the piston approaches TDC.......To get an optimum idle for the lest amount of fuel consumption, timing often needs to be as much as 45-50* BTDC'.
[2] This one is closer to home. MM magazine Oct 2015. 416 stroker. "Initial timing at 1200 went from 15 to 26 while adding an inch of vacuum, Total timing didn't change but a DRAMATIC gain in low rpm throttle response was realised. If tweaked correctly, you will see your idle vac creep up, as we did here''