It immediately became obvious that something was not right. The taper was not correct at the top of the mast. On reflection this could only be due to the stave not being exactly straight, and sure enough on examination it proved to be very slightly curved i.e. the scarf joint was not exactly true.
The only way to mark up the front piece was to draw a centre line on one of the two pieces which formed the stave, and extrapolate it along the second piece to get a true centre line for the front face along the full length of the stave.
I did this and marked up the centre line.
This is what the centre line looked like at station 9 i.e. the top of the mast.
Luckily the curve was slight enough to still allow the front piece to be made. This is what the stave looked like at station 9 after the front piece had been marked up.
There was still plenty of wood from which to cut the front face, so all was still well.
Now I know why the scarfs on the pieces supplied in the timber package are all cut edge-to-edge. They require a lot more wood to make them but you can't go wrong when gluing them up. As long as they are clamped tightly to a flat surface they will be correctly aligned.
I learned something here. From now on edge-to-edge scarf joints will be my norm on wider stock.