[Diy_efi] Timing table change needed with CR increase?

[David Allen]

  Hey guys, I'm rebuilding a 1989 3800V6 from a LeSabre.  It has the SFI 
system using the 1228253 ECM.
Anyway.  The engine has 370,000 miles and the intake manifold gaskets 
started leaking water into #5 cylinder.  So I tore it down for a rebuild. 
Amazingly little wear.  I would like to improve upon this motor by 
installing flat-top pistons in it.  The rest of the motor will repain stock.
  What is a good general "rule of thumb" for changing spark timing to 
prevent pinging when increasing CR?  Will this be enough of a change to 
necessitate spark changes?  What areas of the table will need the most 
reduction in advance? These are fairly highly-tuned from the factory; but it 
only has 8.5:1 compression. I bet a set of flat top pistons would wake it up 
and help on fuel mileage.
  Any input would be GREATLY appreciated!
Thanks,
David