The previous headset, a nice Nuovo Record (which is for sale), had a 13mm spacer.

Steerer tube length 228mm


Head tube length 176mm



So it looks like I need to cut the fork's steerer tube down to 221mm (allowing for the max 45mm difference mentioned above).
In the previous post's picture, the top bearing race / steer tube were threaded down onto the fork as far as I thought it would go. It can go further, but was *very* hard to turn. I've threaded it down a few times and seems to have loosened up. FYI, the threads on the fork are super clean.