First-- I don't understand, I thought soft stretch only came in narrow widths, but then I'm from Canada and ours may be different than yours. In any case it is not good for waistbands. Use a good waistband elastic. Waistband elastic has up and down ridges that keep it from curling over like regular elastic will.

You cut the elastic comfortably stretched to fit your waist. You want the elastic to fit like you would want it on your garment, plus the extra for overlapping. At least that's how you do it for an elastic waistband. The topstitching may stretch out the elastic more.

Now topstitching elastic is hard to do if you've never done it before. You have to stretch the elastic as much as you can while sewing a straight line. Personally I never top stitch. I make my casing just a touch over the width of the elastic and then thread my elastic through. Then I stitch straight up following the line of the front and back seams

Also if you top stitch elastic and you need to remove it someday,that's a lot of work ripping out all that stitching.I do the same as oilpainter,and then i sew an X in the center back,so i know back from front,especially when sewing for kids who don't need pockets .
kathi