Yes, I knew your bikes are old and racy and have thin cranks; hence my comment!
The angle of the taper doesn't matter, as long as both cotters have the same angle. Whatever angle you have, the spindle will rotate to accomodate it. A shallow angle will be stronger, but will require more force to wedge it in place.
Put the cotter through the little hole in the crank arm, without the spindle in place, and push it down to where it looks right, with about the same amount protruding on both sides; now look through the bigger hole and look at the tapered surface from the side. Compare the empty space to the shape of the spindle. You should be able to get a pretty good idea of how thick the cotter should be.
If you hold your two cotters face-to-face, the threads opposed, the round sides should be parallel.
Re Francophile's drawing, Remember that the spindle will rotate in the hole to fit any angle (within a certain range). In the middle drawing, the spindle will rotate a little bit counterclockwise; in the drawing on the right, it will rotate a little clockwise. After you initially tap the cotter in place, try to rotate the crank arms against one another (stomp on both pedals at once, then turn them over and stomp on them that way). Install the nut, but only finger tight. Tap the cotter again, try to move the cranks again, tap the cotter again, test to see if the nut loosened... and repeat these steps until there is no movement. When the nothing seems to move any more, tap the cotter one more time, as is you really mean it this time, and tighten the nut with a wrench. Now you "should be" all done.
OK, I eyeballed it as you say and that view does make it clear that any angle cut is going to engage the flat. Or it will if the pin will go in. When I try to visualize this and get a ‘cut here’ decision it is like doing a triple backflip with reverse twist and the same answer does not come out twice. So must think of it another way. What I can come up with is any angle works so all we are talking about is how thick or thin the cotter is. And then once getting a crank on and having it stay put is accomplished we can work on both cotters, two equal cuts, two equal angles. Making that add up to total correct clearance. But for now all I want is to get a crank to stay on. The next step is to cut the cotter real thin. Will find out what happens. It does look like the pin is going to come all the way through. May have to substitute an old Raleigh arm with a thicker head.
The part about manipulate the cranks against each other is about wiggling the wedge into place, yes? Seems reasonable.