That 1/4" rounded shear seam is a guideline. As long as it's round and not sharp-edged, your fiberglass should lay down OK. I had very good results using the blue good quality masking tape for that step. How evenly you aligned the deck and hull edges at the sheer before gluing will determine how even the glue appears after rounding. I used 3/4" electrical tape to establish the trim line at the sheer with about 1/2" on the hull and 1/4" taped over onto the deck edge. Looks nice. Will be replacing it soon with the more durable automotive type trim tape you can get at an auto parts or marine store.