I have a fork that is not original to a frame, and I really need to make this work. With the headset installed, the top lock nut has enough of the steerer threads in it to make me comfortable with the setup (about 3/4 of the nut is threaded onto the steerer). But this bike originally called for center pull brakes which means I need to add the brake cable hanger to the headset. So . . . how much of the top lock nut needs to be threaded onto the steerer to be safe or adequate? Any suggestions, opinions, recommendations are greatly appreciated.

Tektro make a hanger that mounts to the hole in the bottom of the fork. There might be others available too.

https://www.modernbike.com/tektro-fr...nti-fork-mount

clamp on the stem types from QBP,

or if cantilever center pull brake posts are there ? put on V brakes

The couple times I've had to worry about this - if the nut tightens, I was OK. If the nut would just "spin" where it needed to tighten - then obviously it was not going to work, AND I was screwing up my threads - so don't repeat

Some will tell you three full threads is minimum to make it work. To be clear - some of this will depend upon the manufacturing tolerances between your steerer and selected headset.
[If you look at engineering analysis, even full depth isn't enough]

A Tange Levin can be a relatively inexpensive solution in many cases.

Thanks for the suggestions. I can go with regular side pull brakes if I have to. I also tried a Tange Levin headset (nice classic look), but had to switch to the cheaper Tange Passage to gain a few more mm in stack height. I can get three full threads pretty easily. Again, thanks.

The rule of thumb is to have a minimum of three full turns of engagement for the locknut. For a standard 24 TPI headset, this equates to 1/8" engagement.

Paul makes a version of their Funky Monkey Cable Hanger that clamps onto the vertical part of the quill stem. A bit pricy ($40-50) but should be the nicest cable hanger you're going to find. Get the 7/8" version for a quill stem.

I've used the Tektro fork hole-mounted cable hanger before and it works well, but might not work depending on what else you are using the fork hole for. With centerpulls it might not work as I believe the Tektro cable hanger has a recessed hole for the mounting bolt.

Agreed, +1

IMHO, as a lifelong mechanic/technician including 25+ years professionally as a fomoco and ASE Senior Master certified, any minimum rule of thumb should be considered under ideal conditions, good to very good threads that match closely, ymmv.