Where would the front castor be measured from?

[Blue Chips]

Tri gear, front wheel

Working on a friends CZech Sports Cruiser, he has it completed but for substantial body shake/shimmy upon applying brakes.
This is a known problem with an ad to install two belleville washers in the front vertical spindle (item# 26) and limiting tire pressure (done).
Rear brake rotors dial true.

Improper castor comes to mind as a frequent cause in non aircraft related builds but I cannot figure where this measurement would take place (in degrees) here.

Would this be measured in the vertical spindle or the trailing Fork assembly (item #9)
In virtually all pictures seen of this assembly this fork is quite angled as depicted in the attached pic, his is all but horizontal.

opinions? open for any.

Thanks
Ken