You might not be doing anything wrong, When making dresdens, the primary number that you need to know is the number of degrees in each wedge, There are 360 degrees in a circle so if you have 10 degree wedges, you will need 18 for a half circle, if the wedges are 18 degrees each, you'd have 10 in a half circle, and so on. Standard rulers I have are 10, 18, and 15 degrees plus one more that I don't remember. If you have a 40 degree wedge, it would require 9 segments for a full circle, and an exact 1/2 circle would not be possible with this wedge as it would be 4 wedges totaling 160 degrees or 5 sections that would equal 200 degrees. If the size isn't marked on the template, take a piece of newspaper and continue to trace your template into a circle, overlapping the seam allowances, until the circle is complete. This will tell you how many wedges you need for the full circle. If the number is even, great, you can make 1/2 circles, of it is odd, you cannot, Is this plain as mud? haha As for sewing the pieces together, sewing pairs to pairs prevents distortion.