Basic starting point for the guide is 14 mm from the hexagon nut to the edge of the guide rod. Most of the starting measurements for the front end are in the Factory Service Manual. When I rebuilt mine, I just set everything to the factory default: tie rod length, drag link, supporting rod, eccentric bolt. These all have "factory neutral" settings from which to adjust. The guide rod is 14 mm. There's even a pic of the "self-made tool" to start with.

http://www.circletrack.com/chassistech/ctrp_0801_caster_camber_settings/photo_02.html

The upper ball joint on the UCA is behind the lower ball joint, which is near coincident with the spindle, so as you discovered, moving it forward should increase caster.

FYI: the UCA isn't attached to the frame, there's some fat rubber bushings in there that go bad. That's why I asked. If the top is wiggling around on old rubber, then the caster can't get set. Since yours are new, not an issue.

Jumping right into alignment specs on a W123 is a bold move. No one even used the term "spreader bar."