The Austin’s suspension is somewhat lower than standard so the castor angle probably needed a tweek but how to measure it? Initially I thought I would use some flat surface on the axle or the stub axles but are those surfaces perpendicular to the king pins?
The jist of it is with the wheels straight ahead and sitting on a level surface, place the level on something vertical (it’s magnetic) as in the picture and zero it. Turn the wheel 20° right and record the result then go back to straight ahead and go left 20° and record that number, in my case it was 1.3° for right and 1.2° for left so to calculate the castor it was 1.3 + 1.2 =2.5 then here is the magic part x 1.5 = 3.75°
Evidently, it’s important that the wheels are moved only 20° each way and that it’s related to the 1.5 that you multiply the result. I will have to go ask a Maths teacher (Ian?) as my brain hurts when I try to do the trig.