Sorry, but I have not yet gotten a good neg for carbon from HP5+. While I suggested D-19 to others, the suggestion was based on my failure with my usual 'high-energy' developer (Ilford Universal PQ), and the opinion/testing of others. I just decided that I won't use HP5+, as it is not the best choice for me.
But if I were to try HP5+ again, and did not have D-19 to try, I would try straight Dektol, 68F for 8 minutes for a HP5+ negative of a scene that has a range of at least 7 stops of light in it (detailed shadows placed on Zone II to III). I would not bother exposing the film with a scene brightness range of 5 stops or less.