I’d do the swatch in stockinette stitch and pay special attention to row gauge since you want to center the B over the 12.5 inches. Usually, stitch gauge is most important, but since the B needs to be in the middle, you need to know how many rows you’ll need to knit to get to 12.5 inches most accurately. Then you can see what stitch gauge you get to know what to cast on.
For the back, you can cast on the same number of stitches and compare them after a couple of inches. They should be quite close.
Are you using a pattern? If so it will usually tell you what stitch you should knit the swatch in. If not, I would do two separate swatches, one in stst for the front and one in garter for the back.