If you quilt it yourself and you are happy with the results, then continue as you have been doing. When I did mine on a domestic machine, I usually went with the same amount as you. I didn't use quite as much starch as you mention. The extra fabric gives you something to hold on to when the quilting gets close to the edge.
If you take it to a long arm quilter, however, they need the additional fabric to attach it to the frame.