|I'm always frustrated when I top-stitch a turned up hem - either it doesn't properly catch the turned up fabric on the inside, or it does, but it's wavy and not parallel with the edge of the fabric.
So, here's a trick I discovered:
First turn fabric to wrong side and stitch along the very edge of the turned up hem. Your goal is to secure the fabric as close to the raw edge as possible.
Note: the line of stitching in the foreground is the serged edge. The hem is secured where the needle is - right at the raw edge.
Next flip the fabric over, and using the line of stitching as your guide, top sew with a stitch of your choice. I prefer a wide stitch that will hide the original line of stitching and allow me to "fudge" the spots where the hem might not have been perfectly parallel with the bottom edge of the hem.
I find it works best if you perfectly match the thread in the original line of stitching with the fabric, and then use a contrasting wide stitch when you topstitch.
Note: it's a cuff - you can see in the foreground the part I've already top stitched (satin zig-zag), AND you can see the line of stitching I'm using as a guide.