I have exposed HP5+ at EI 100 (twice in the same month!). I found the time for it at EI 160 on the massive development chart to be 20 minutes so I reduced this to about 16 minutes and it seems o.k. I would expect somewhere between 12 and 14 minutes to be about right for EI 64.
Massive Development Chart: http://www.digitaltruth.com/devchart.html
One of mine with HP5+ at EI 100: http://www.apug.org/gallery/showphot...8&ppuser=12229