Block the swatch the way you'll wash the finished piece: same soak, same drying, no pins if you won't pin the garment. Knit it the way you'll knit the garment too; a swatch knit flat can come out different from a body knit in the round.

Measure the swatch before you wet it. It's what you'll see on the needles, so Convert can tell you what to knit to.