Okay, so I’m still in the hobby.
All I have to do is figure out how to transfer the exact quartering angle from the front wheels to the back. The trick is that there is almost no datum – no way to get a good handle on the centre of the axle.
It – the axle centre – should be really close to the centre of the tyre. So my first thought was to make two cups that fit on the tyres exactly, drill out holes for the crank pins, and slip them over the wheels, and mark them. Then I could slip them over the rear wheels and line up the marks.
I got about half-way through turning the first cup, and was about to fashion a longer boring tool, when I realized I had fallen for that old adage: when all you have is a hammer, everything looks like a nail. So I slept on it again.
Today I remembered that The Girl received some Polymorph beads from her uncle a couple of years ago. So I asked her if I could use them. She said okay, but warned me that they would want to stick to metals and tear them apart.
Best be careful then!
So I ran a little experiment this evening. I found that it doesn’t stick to brass or steel very well, but it sticks extremely well to the resin wheel centres. I also found that it wants to shrink by about 1%. These are all good things to know when designing a tool.
Or I could just look through the spokes and try to do it by eye.
Pembroke:87 
Can you coat the resin with something to make it not stick? Vasoline? Or mold release?
Craig
I’m probably just going to mask it off. I don’t need or indeed want to make an impression of the spokes.
“Or I could just look through the spokes and try to do it by eye.”
Well, that’s all that Trevor Nunn does, and you don’t get any better at loco building than that. (On seeing Trevor’s scratchbuilt S scale GER 4-4-0 with working inside Joy’s valve gear, Iain Rice muttered to Tim Watson, “Modelling like this is when we get down on our knees in reverence.”)
LOL. I am going to attempt to make up for a lack of such skill with a tool! Let’s see how it goes.