Having been up close to 3 Tartan 27 rudders, I do not believe they were custom fitted. They seem to be made to the plans, and very simple. I think this has already been said, but there seem to still be much conjecture about other factors, which seems to create some confusion. I hope this can help.
The only essential feature for correct alignment of the rudder is that the heel of the rudder post is in a direct line with the shaft in its tube. Look down the steering tube and see if the heel isn't exactly in line with the casting at the bottom of the keel into which it must fit. It should be. So, the only other dimensions needed to fabricate or repair an existing T27 rudder are: 1) the length of the rudder blade itself between the shoe on the keel and the tube in the hull above, and 2) the size/shape of the aperture for the prop. It really shouldn't be any more complicated than that. (Even if the rudder were completely missing the only other metric needed would be the lateral area.)?
If the angle you are referring to is the angle the shaft takes to establish the aperture, this is much more arbitrary in that it is not critical to alignment at all, only clearance, so close approximation is fine. Don't get me wrong, large amounts have been written about apertures, but these are irrelevant to making a responsible repair to a T27 rudder. (For a completely different perspective on that, look up AtomVoyages, where he sings the praises of dispensing with the aperture altogether in favor of a well hidden auxiliary outboard in the lazaret!
Bottom line: I think if the heel of the rudder where it meets the keel is in a direct line with the shaft above it, then it will function correctly. It shouldn't have any angle and it shouldn't be any more complicated than that.?
Best of luck!
Alan
