If I'm remembering correctly, the HPX171 can sync timecode through firewire only, it has no other timecode in/out capability. So you'd put the HPX250 in DVCPRO-HD to match the HPX171 (note -- NOT 720pN!) and then you can sync the 171 to match the incoming timecode being fed by the HPX250. I laid out the procedure in The HPX Book.
States in the HPX Handbook " Also, make sure the 1394 TC REGEN is set of OFF. Finally, switch to MCR/VCR mode....."
I could not get to the "1394 TC REGEN" selection until I entered MCR mode. Couldn't do it in CAM mode via RECORDING SETUP.
But that is for my particular model HPX171e and maybe different than others.
I was easily confused about the "(don't worry about setting the TC PRESET menu item)" explanation. It just doesn't matter what you enter there as long as you hit OK... then the new generated TC comes via Firewire from the MASTER Cam. It's simple.