I have shot a lot of Harman Direct Positive portraits and controlling it is a slippery business indeed. Here's where I'm up to:
I use a 2 stop orange filter for portraiture to overcome the truly vile ( coarse, ugly, contrasty) orthochromatic rendition of skin tones. My effective EI is 1.5.
Preflash is critical. I use 0.8 seconds under a 4x5 enlarger (150 watt lamp) set high with f16 on the 150mm lens. A preflash of 0.7 seconds or 0.9 seconds is different not only from the point of view of contrast control but the EI of the paper changes too! Testing and optimising for your own system is the only way forward. My first 50 sheets were "wasted" in testing.
Preflash reduces the capacity of Harman DP to deliver a decent black. I use warm fresh Dektol 1+2 to get as much density as I can.
Fixing is demanding. Weak fixer will leave a nasty magenta stain in the paper. I use Hypam 1+4 fresh and warm.
Exposure is deadly critical. 1/3 of a stop makes a visible difference. During a portrait session I have an assistant running to the darkroom with exposed sheets and calling out corrections as the shoot progresses.
Harman ostensibly produced this material as a resource for pinhole photographers. With the limited control available in that genre of photography I cannot see how anyone can get predictable results. In the studio with everything under control Harman DP can yield unique camera-original pictures.