You pick up in every stitch along horizontal edges, such as the back of the neck. For vertical edges it's more like 3 stitches out of every 4, and for a curved or slanted edge usually 2 out of every 3 works. If a vertical edge is garter stitch, then you pick up 1 out of every 2 sts. More or less - sometimes you have to just try it out and see what ratio works according to your stitch to row gauge.
sue- knitting heretic