I'm more or less in the Jnanian camp. I would take your outdated HP5+, find a scene with a reasonable range of zones, and expose 3 sheets: 320 ASA (my normal rating for fresh HP5+), 200, and 100. I would tray develop in HC-110, but I would use dilution H (1:62) to get a time somewhere around 11-12 minutes (I like longer times because I shuffle process, but that's with 4x5, not 8x10; if you are doing one sheet at a time, use dilution B, 1:31). Then see which sheet looks best to you and that's your rating for the old film. I did exactly this with two boxes of very old Tri-X I found in the back of a drawer, expiration dates in the 1980s, and an old bottle of HC-110. I ended up with ASA 50 but your film is fresher, and faster than 4x5 Tri-X in the base case.