It sounds to me like you just want to do radiating lines from a central point. So your scallop is essentally a 3/4 round circle. You want you lines to start where an element in your quilt (probably inside borders) make a 90 degree angle.
When I do radiating lines like that I use a protractor so I can space my lines evenly at certain degree intervals. I find 15 degrees work well. So line up the zero on your protractor with the point of the 90 degree angle and make a registration dot every 15 degrees. Then take any simple ruler and mark you lines. Quilt along your marked lines. You can pick up a protractor in any office supply store or even in the school supply section of Walmart. You shouldn't pay much more than a couple of dollars for it. You can get a full circle one or a half circle one. I have a full circle one like this: http://www.officedepot.com/a/product...rotractor-360/
Look at the 3/4 circle fans in the corner of this quilt. That is how marked the radiating lines in these fans in the outer border of this quilt, which is essentailly the same idea as your sunburst only you wouldn't do the curved tips just quilt your lines all the way to the quilts edge.