Sometime can you explain gauge ratio?
This is actually simple, but it sounds complicated and some people tune out because they think it requires math.
Here's an example.
I'm knitting a sweater. The gauge is 5 stitches an inch, 8 rows an inch.
That tells me for an inch of vertical edge I need to pick up 5 stitches for every eight rows. (Five stitches to eight rows is the ratio.)
Now I need to pick up stitches along a mostly straight armhole with a curve to the underarm and an underarm section that's straight across.
For the vertical, straight, edge plan a pickup pattern. To pickup five stitches in eight rows: pickup, skip, pickup, skip, pickup, pickup, skip, pickup. Repeat until done.
Other skipping patterns work just as well if skips are kept to one at a time. Try to space the skips as evenly as possible and then don't worry about it. It will look fine as long as the pickup doesn't pucker up or spread out, which it won't if you're picking up five stitches an inch (8 rows).
If it doesn't offend the style of what I'm knitting, I like to purl the first two rows after the pickup row. The purl stitches make a neat looking edge ridge and they pop up over the picked up stitches to cover the pickup pattern.
The curve to the underarm is a bit more art than science. Eyeball it and guess how many inches it is. I usually pick up a few less stitches than I think I need just because it's a curve.
The underarm is horizontal. Pick up every stitch.
I often leave the body underarm stitches on a stitch holder so there are live stitches to knit into. Why have bumpy bound off arm pit stitches when there's no need? But caution doing this. It works great for a firm fabric, maybe not so great if the garment is knit loosely.