Instead of using a set formula for the increases, I drew out a chart to make sure the shape is what I wanted. I wanted the increases to be worked over 2", which according to my row gauge is 16 rows total.
So I got out the trusty graph paper and started drawing. Here's what I came up with:
So I'm increasing a total of 11 stitches over 15 rows (1 stitch every other row 4 times, then 1 stitch every row 7 times) - close enough to my 16 total.
After working my underarm increases, I now need to figure out how many stitches to cast on for the sleeve at both ends of the back.
I want the sleeves to be about 10 1/4" long from the shoulder to the cuff. So 10.25 x 2 (sleeves) =20.5" + 9" (neck) = 29.5" wide. Then I subtract the body 29.5" - 18" = 11.5" for the underarm seams. But I have 2 underarm seams, so I need to divide by 2 = 5.75" for each side or 31 stitches wide (5.75 x 5.5=31).
I already increased 11 stitches for the underarm shaping, so I can subtract that from the 31, so we have 20 stitches to increase.
But before I cast 20 more stitches on for the sleeve, I need to make sure the total number of stitches follows our stitch repeat numbers (7 sts + 4). 99 sts + 22 sts + 40 sts = 161 sts total. 161-4=157, 157/7= 22.428...... = not going to work! So I messed around a bit with the numbers and figured out I needed to add 15 sts on each side (99 + 22 + 30=151 sts. 151 -4=147, 147/7=21 repeats). The sleeve will be about 1" shorter on each side, but having the numbers work out makes it easier to place the repeats correctly so I can live with that.
After casting on the extra stitches, I was able to veg out for awhile and knit even for 7 1/2". Once I hit that point, it was time to think about the shoulder shaping.
There will also be a bit of back neck shaping involved which will involve separating the back at a certain point, but I think it's safe to say that I'm pretty sure you just want to see a progress picture by now:
Next up: Math for a cardigan front and neck decreases - fun!
Thanks to everyone who made a suggestion in my time of crisis! I decided on the Rowan DK Wool from elann.com for the baby goods. I love Rowan and hey, it was cheap! If it wasn't a budget project, I probably would have gone for the Zara, but I'm sure there will be a next time!