Forget using a regad for a stitch groove keep it for edging grooves. Many of us use a pair of dividers or a screw adjustable crease to get a stitch line. I prefer to use a groover such as the one below. With a bit of practice you can get the stitching to be just below flush which stops any wear or use a smasher (polished hammer head) to set the stitching so it’s flush to start with and settles to about 0.25mm below the surface of the leather.
https://www.tandyleather.eu/en/product/craftool-pro-stitching-groover-set
https://www.tandyleather.eu/en/product/craftool-adjustable-creaser
I am in no way connected to Tandy. Just easy to use for reference.
