Adrian, movement at the crank is twice the movement at the camshaft. I'm never sure which is the half-time sprocket - I try to refer to them as cam or crank.
28 teeth on the cam sprocket - so 360/28 - nearer 12.9 than 12.8 degrees. Therefore nearly 26 degrees - 360/14 - per tooth at the crank.
I'm working through this for my own benefit as I will get to the problem eventually.
I have a crank sprocket - probably from Bruce - which has a fourth keyway. Unfortunately it's not to hand.
The next question is how to assess the positions of the keyways relative to the teeth on the crank sprocket.
I don't possess a timing disc, but that might be used to measure the positions. Given your set of measurements for one keyway it should be possible to work out which of the other keyways would be more accurate. And then because this is a situation where your answer is right or wrong depending on one slip of logic you find it should be the other one.