Here is a method that I use that helps you get the true length of all the lines you need. I would draw the top view of the fitting. Mark the 4 corners of the square to round 1.2.3.4. then section off your cirle into four quardrants, and divide those quardrants into equal sections ( I would divide it in 1510 degree incremants what ever you perfer). Now go around your cirle and mark each point a,b,c,d, ect. Now since the top and bottoms are parallel that distance is constant for all the corners. So this is what you do, take the hieght of the fitting ( or the base to the top of the square to round) and mark that distance on a vertical line on a scrap pc of metal. Then draw a line perpendicular to that line. Now from each corner of the square use your dividers to to go from the corner of the square to each letter section on the cirlce. Do this for all the circular sections in each corners quadrant of the circle. On your scrap pc of metal, mark each lenght on the horizontal line, from the intersection point of the vertical line and the horizontal line. (make sure you mark each letter on the scribed mark you make) now from the mark you put on the vertical line to your marks on your horizontal line is your true length of each line segement. Just like a center line square to round except since it is no longer symetrical you have to now do each quardrant. The tricky part now is to figure out how it should look once you lay it out. That is allways the real tricky part for me. Good luck, and I agree with the hammer, when in doubt triangulate.
