For HP5+@320... My Personal Normal(N) Dev time for ID11/D76 Dilution 1:1@20C is 9min. My 2 stop contraction Dev time(N-2) is 5.5min. You'll probably be somewhere around here but what you do is really at best a guess. The Subject Brightness Range(SBR) of the original scene will play the determining part ie if it was a scene that had bright highlights they will most likely be blown out, but the contraction of development will help a bit. If it was a Flat low contrast scene you will most likely end up a flat negative. Higher grade printing might help here.

Good luck
kev