Paper Scaling Vs Trig

So I used the scale printout for the rudder section to find the half angle for the rudder construction jig. At first I just took measurements and used trigonometry to find the angle. I got 31.6 degrees. I then laid out an extension line on the printout and measured it on paper with a protractor. Fully expecting my trig to be verified. It wasn't. By laying it out on paper I got 34 degrees. Then I looked at my measurements again and realized that very small difference in what you read off the scale make a huge differences in the angle calculated.  So I printed out a scaled up version and repeated, this time I calculated an angle of 32.6 and measured 33. I'm going to go with 33.