Spark tuning issues. Please help

Hi All,
I am really battling with knock (L98- E38). I have calibrated my MAF and VE to around 1% accuracy on LTFT. My car was dyno'd for WOT –P.E. and my spark in the high 0.50 APCYL, is fine. But in the cruise and lower airflow (2000 -3200rpm) areas, I am continually getting knock. How low can I take the spark in this area (0.2-0.5 APCYL). I have already reduced some area by 10% and have seen an improvement. I also noticed a reduction in valve temp and a smoother, with more response when I want to accelerate, but how far can I go

Regards

Mark