Any errors in instructions not in errata?

I'm trying to make the Classic Shirt, View A and the instructions for the 'Wearer's Right Set-In Placket are more than confusing. It says trim 1-1/4" from right front shirt edge, then sew a row of basting stitches 1/2" from one long edge. Then I'm suppose to add 1" x 29" of interfacing using basting stitching as a guide. Then fold along stitches and sew the placket back to the shirt front using a 1/2" seam allowance. This would leave me with a placket that's 1/4" wide (and interfacing hanging over). That can't be correct...can it? I've read the errata and looked at photos of completed garments. I still can't figure out how I should do this. My husband checked my math and he thinks 1-1/4" - 1/2" - 1/2" will leave 1/4" every time. (He's better at math than I am but that was my conclusion as well.) I feel silly and not very bright writing this but I don't know what to do. I've already cut it also... Help, please. bj canerday