help with idle timing in park b0950

im fairly new to duramax tuning but understand the basics. i have been having a problem with a rolling/unstable idle only when warm and in park or neutral. foot on brake or A/C on makes no difference, frp stable and matched desired. injectors and fpr are new bosch units. im seeing in b0950 it calls for -5 degrees of timing when when above conditions are met. i think this might have somthing to do with the idle problem. i plan on advancing it a little (in b0950) but have some questions. other than noise, why would GM want the timing that retarded under these conditions? also what would be a good starting point for agjustment? 04.5 lly auto with 200k miles. thanks, i appreciate any help/thoughts.

so I guess the lly section is kinda dead nowdays..LOL. I went ahead and changed timing from -5 to +2 degrees in b0950, at operating temp 6-800 rpm. it make the idle quality much better with hardly any increase in noise. I will continue to monitor. Hope this helps somebody someday.