here’s what I would do:
there’s no need to caston ALL those stitches for a swatch in the round–esp[ecially since you might have to do a few of them before youget the right guage. On your circs, cast on 20 stitches, knit, but at the end of the row, just slide the knitting to the other end of the circular needle, and bring your yarn around back–you’ll have a big strand running arouss the back, and knit the next row, slide to the other end, bring yarn around the back again, knit, etc. That will simulate your circular guage better than a flat st st piece and you won’t have to cast on all those stitches for knitting in the round.
The pattern says, for guage:
you’ll need approx: 250-300 yds of a yarn that works a gauge of 4 sts per inch on a US 8 needle (but you will work the top with US 10.5 or 11 needles)
So I would swatch with size 8s, and then change to size 10.5s. If the 8s don’t give you the 4sts/inch, go up or down a size until you get the right guage, and then use a needle 3 sizes bigger for the pattern.
To change the pattern for knitting in the round, you’ll have to subtract 1 stitch from each side of the front and back (this would take up the seam). Then, you’d knit it on circs until you got to the armholes, at which point you’ll put the back on a holder or waste yarn, knit the top of the front in sts straight, then so the back straight as well. Don’t forget to mark where the side seams WOULD be when you CO, so you know where to put the decreases. When i do this, I use a RED marker to mark the beginning of the round (red = stop, so stop this round and start a new one) and a green or other color (green = go, so keep going around) at the opposite side.
So I would:
CO 19 sts, PM, CO 19 sts place red marker, join
Knit for 16 rows… on rows 12 and 16 you’ll have to decrease BEFORE and AFTER each marker.
Work straight until it measures 9 inches
After that, divide for the arm holes by placing all the front sts on a holder or waste yarn, and working the rest as written, and doing the same for the back.