Mr. Pheonix,
Use your bowsquare, and measure 3/8" to 1/2" above shelf height....take some serving cord (kite string, dental floss, etc) and do a double over hand knot on the bowstring where you want the nocking point....wrap both ends of your serving/string around your nocking point four or five times doing an overhand knot each time....finish with another double overhand (aka surgeons' knot).....put a dab of glue, or if acrylic, apply heat to melt the material just slightly. Trim to suit your taste. You could put a second knot above your arrow leaving a space just right for your nock.