Hi Tony! I'm planning on doing a video around the subject soon as more people have been asking these questions. Keep an eye out for our Instagram page.
It boils down to this: I'm preflashing using the diffuser spot (on the Pinsta Pro) for about 10 seconds with a 1.000 lumen LED flashlight. If you don't have one (or own a Pinsta Go), alternatively you can download a picture of a pure white rectangle on your phone, put the brightness of your phone up to max, open the hole of you camera and place the screen of your phone against thr camera, in front of the hole, for about 10 seconds. Experiment with the times.
Preflashing will
A. Broaden the dynamic range of your paper, making Harman Direct Positive paper a bit less contrasty
B. Shorten the exposure time needed with about 30% (my experience)
It's all about experimenting until you find something you like.
Hope that helps!
Martin