I try to have my keepers be 6.0 or better, and I really like 7.5s when I can find them (all the beauty of a VF book and cheaper than the aforementioned price spike at 8.0). Anywhere in that range is high grade enough for me.
7.5 to 8.5 is my high grade for many early (59-61) PHM issues. PQ is important as well. Rather have a nicely centered, White page 7.5 than a Cream 8.5 that only shows it as an "ANGE TALES" issue.
Later PHM issues (62-63) are sporadically available at 8.5 and higher. But there is a big price curve for anything labeled in the top few price grades as was mentioned earlier.
Seeing a great deal of alternative company high grade PHM books in the last few months as well, so keeping track of the data has been more challenging.
Either way, patience is a virtue with these critters
-bc