Then along came Elizabeth Zimmermann and I was liberated from all this nonsense. Starting with her words of wisdom and with many years of experimentation, I've devised my own no-fuss set of pick up rules. Here they are, and I hope they will help you too to relax over this important aspect of finishing a garment. (Caveat: garter stitch is a separate category with its own rules. That's for another day.)
1. With a garment in stocking stitch or in a textured pattern (other than garter stitch), make sure you have one clean column of stocking stitch along the pick up edge. If necessary, add a stitch at the edge of an aran or other textured design to create a selvedge for picking up down the road. Knit this column on the right side, purl on the wrong side.
2. When it comes time to pick up, focus on the inside half of this outer selvedge stitch, this one here:
Example: In the double wave cable jacket I'm currently working on, there were two stocking stitch columns at the edge of the armhole, just as in the drawing above. I worked into the inner half of the outer column, leaving one column whole, right next to the sleeve. See?
3. What about pick up ratios? On front borders I usually pick up 3 stitches for every 4 rows. On diagonal edges, like a V-neck I pick up every stitch. For armholes, I usually pick up 2 stitches for every 3 rows, even though the edges are straight like a front edge. I guess I like the armhole to draw in a little. Of course, if you have live stitches, as you might across the back of a neck, then just knit those live stitches. At corners, where there tends to be a gap, pick up the horizontal strand between stitches and work an M1 into it. Make such M1s lean toward the centre back (in the case of a back neck border) or toward the underarm ( in the case of a sleeve). Very tidy.
Sometimes you have to change the rules. For instance, on my Perth Cardi I picked up 3 stitches for 4 rows all along the long diagonal front edges to prevent them from drooping. The key is to remain flexible. If a particular ratio doesn't look right, try something else.
4.Oh, and generally, I go down one needle size for borders.