I've been working on my "Make it Up As I Go Along" alpaca sweater when it came time to make sleeves. I pulled out Maggie Righetti's "Sweater Design in Plain English" (hereby referred to as SD), to try to figure it out. Now this just happens to be the reference book I own, but I'm sure there are many fabulous sources out there. SD pages 256 to 262 deal with planning set in sleeves. Also, I made sketches to show what I did for my sweater, sorry I've got no neato graphics software so I hope you can read my scrawl.
Underarm Curve Planning:
Now I'd already made my front and back by looking at other knitting patterns and eyeballing my sweater and decided to bind off 5 stitches/4 stitches/1 stitch to make the underarm curve of my sweater body. (My gauge is 5 stitches to the inch.) I already KNOW that my sleeve underarm curve needs to match this body underarm curve. I also KNOW that my armhole depth is 7.5". See armhole depth sketch.
How deep should the sleeve cap be?:
SD shows that the sleeve cap must be shorter than the armhole depth by the amount of the "flat part of the top of the shoulder". SD gives this standard, arbitrary measurement as 3" for an adult sweater. This means the top of the sleeve cap (that last bit of stitches you bind off at the end of the sleeve is about 3" worth of stitches). This flat 3" is centered at the shoulder seam, so you'll have 1.5" in front of the shoulder seam and 1.5" behind the shoulder seam. What's that got to do with sleeve cap depth? My armhole is 7.5" long and my sleeve cap will need to be 1.5" SHORTER than the armhole depth for the curved sleeve cap to fit into the straight armhole. SD has a drawing on page 258. My sketch is here.
Sleeve cap calculation recap:
Sweater armhole depth: 7.5"
MINUS Half of sleeve top: 1.5"
Sleeve cap length: 6"
You want to make the flat bit at the top of the cap PLUS the sleeve cap depth equal the length of your armhole. If that doesn't make much sense to you think of the big picture:
Front armhole depth: 7.5"
Back armhole depth: 7.5"
Total armhole depth: 15"
Front sleeve depth: 6"
Front half of flat top: 1.5"
Back half of flat top: 1.5"
Back sleeve depth: 6"
Total sleeve curve length: 15"
Figuring sleeve cap Decreases:
Okay what do you know so far? I know I will be start decreasing the sleeve to match what I did on the underarm of the body of my sweater, namely 5 stitches/4stitches/1 stitch on each side. I know I want to end up with about 3" worth of stitches at the end of the sleeve. I'm making short sleeves, so I measured my bicep where I wanted the sleeve to fall. My measurement is 12" at 5 stitches per inch is 60 stitches plus a few stitches for ease and seams, I came up with 66 stitches to cast on. I decided I wanted to make my sleeves 1.5" long before beginning the sleeve cap decreases.
Cast on 66 stitches
Want to end up with about 15 stitches
My body armhole is decreased 5 sts/4sts/1st on each side which is 20 stitches total
66 sts minus 15 sts to remain and 20 sts from the first decreases leaves 31 stitches to get rid of in the cap.
SD gives various methods for figuring out row gauge and finagling those decreases. In my case I figured I'd have about 5" to get rid of those 31 stitches and my row gauge is 7 rows per inch (35 rows to work with). If I lose about 1 stitch per row I'll be just about right. I simply cast on 66 stitches, knit straight for 1.5" and began casting off for my armhole curve. 5 stitches each side once, 4 stitches each side once and 1 stitch each side until the sleeve cap was about 5" long. I kept measuring the cap against the pinned together armhole to make sure it was fitting and the curve was working out ok. I wanted my sleeve cap to have a more curve at the top than a flat 3" and bound off 2 sts at the beginning of the next four rows then bound off the remaining 14 stitches for my flat 3". Since I'd been measuring it to the armhole during the knitting of the thing, I knew it would fit and pinned it in (photo above) as proof. See sketch for sleeve diagram.
See, not so hard to make sleeves. You just need to remember to make the caps shorter by that crazy flat shoulder factor. I really helped me to make sketches and think of the big armhole picture. Good luck.