Just got my new On One 456 Summer Season built up and took some time measuring angles and stuff for future builders.
The frame is 16" and fitted with a pair of old but plush and reliable Boxxer 151 forks 6" forks.
First off I measured the lower fork crown race face to the centre of axle and compared it to my sons Marzocchi DJ3 100mm forks.

Boxxer = 520mm
DJ3 = 495mm

So a Boxxer 151 is 25mm longer than a 100mm DJ3.
This is due to the Boxxer having a thinner/lower lower crown.

The head tube angles are as follows:

66 deg with fork at full extension (no rider)
67.5 deg with 40mm sag (with rider)
72.5 deg with fork at full compression (springs removed).

So Brant`s predicted 1 deg change of head angle for every 1" of travel holds up (did I ever doubt him).

It also shows that a Boxxer 151 will give 150mm of travel for a 25mm gain over a 100mm fork.