Stand development is definitely going to be the way to go here. I'd try 1:100 for an hour. 15 seconds of slow agitation, then swoosh it (like you would a glass of fine wine) three times at the 20 and 40 minute mark. Temperature doesn't matter too much with stand, but keep it in the 20 degree celsius range. I've never done HP5 with stand, but I can imagine it'll work similary to Tri-X.