OK, I've got a kind-of solution to the hole-measuring problem - I've marked the chuck with 10-degree intervals. Being a 3-jaw chuck it already has marks every 60 degrees, so it wasn't too much hassle to make more marks in between. The hole in question is not super-critical - opinions differ as to what the steering arm angles on ackermann steering should be anyway, so it doesn't need 1-degree accuracy. This is a stopgap, I'll continue the theory of fitting an angle gauge to the back end of the spindle and having a pointer to align it with. The one I'm doing right now will suit OK at 20 degrees, I hope, and that I can do with my current trick.
Other holes: I make an axle for the wheels. It's made from a piece of 20mm BMS which is drilled to take a skewer through the middle and also for lightness. I've just done one: it has an 8mm hole through the middle and then stepped holes (easier than a taper) in 9, 10, 11, 12, 13, 14, 15 and
16mm to decreasing depths - the taper is off-set so that the thick part of the axle is not in the middle. Out of interest I weighed the 124mm length of bar, with no holes, and the one I've just drilled: 10.5oz. and 7oz. respectively. However, since it needs a hole in the middle (minimum 6mm but it's easier to drill 8mm) so I don't really save 3.5oz. per axle... I doubt the saving in weight is worth it, TBH, but the few people who take it apart and look at it and have the knowledge to appreciate it should be impressed.