Looks like Chris might be able to help me out!

Here's a selection of pics that I took to help dimension the new spacer:

The lower 52mm bearing cup of the frame:


With the bearing installed:


With the original crown race/baseplate sitting on the bearing:


You can see here that the headset/frame is designed to have a semi flush (+1mm) fitment with the fork.

A point I want to add here: with the crown race/baseplate installed on a 1.5 steerer (at the 1.59" section), this particular race opens up to an OD of 52.2mm, which is larger than the 52mm opening in the bottom of the frame. This means, with the fork test fit, it rubs! Its a little bit of a bummer--I thought at first that I just hadn't torqued down the headset, but on closer inspection/measurement it was clear the race was rubbing the frame.

Here's the 1.5"  carbon headset spacer installed as a test:


This test confirmed for me that adding 8mm (underneath the baseplate) as a shim is just barely enough to clear the fork knobs. Since I'm having a custom part made, I'm actually going to go +11mm instead of +8mm to give everything a touch more room.

On my steerer, the 1.59" OD pressfit section is just tall enough to grab after an 8mm shim:


Another 20 min in Solidworks, and here is the drawing for the baseplate adapter I hope to have made with MTB Tools :

Have one question:  Since the steering tube is tapered and your spacer plus the crown equals around 12+mm, how is your bearing making good contact with the tube since the ID goes down to around 40mm at the point where it makes contact?  I've tried with the Niner's 5mm crown and noticed after tightening the headset, the shock would wobble at the bottom due to not making perfect contact between the tube and bearing?