Prism99

I would maybe try marking the tape line, then removing the tape before quilting. After placing the tape, I would try using a long ruler and a sliver of leftover white soap to mark (old, dried out scraps of soap work best). If you don't have soap slivers, then white chalk would probably work. If possible, I would sharpen the chalk in a pencil sharpener.

I haven't used painter's tape as you describe, but am wondering if the tape may not be sticking because of starch in the fabric (or sizing, if the fabric was not washed).

Edit: I also think the tape stays in place best when the quilt and batting are quite flat. When I Googled, all the photos I saw were on very flat quilts -- such as those made with Quilter's Dream needlepunched cotton batting. There would be less flex in the quilt sandwich with a thin, stable batting; more flex with a fluffy, polyester batting.