In case you’ve not seen this neat way of making a swatch for an in-the-round project, here’s how to do it.
Use a pair of double-pointed needles. Cast on the required number of stitches. *Don’t turn the work, but instead push the sts back to the other end of the needle. Draw the yarn loosely across the back of the swatch. Work the next row as if it is the next round. Repeat from * for the required length of swatch. Cast off. Cut the strings at the back. Block and treat as the project is to be treated. Pin out and measure.
As with all swatches, measure across the central sts as the edge sts may be distorted.
Here are pics of a (small) swatch worked in this manner (a) from the front; (b) from the back and (c) pinned out for measuring with the strings cut.
Note, if the yarn is slippy, you may have to secure the edgemost stitches so that they don’t unravel. You can do this with a sewing machine or by crocheting the edge as you might do with a steek. It helps to have long lengths of yarn at the back.