Use stitch markers. When you are swatching throw one on about 1/4 of the way in on the row, and then another 1/4 from the end. Note how many stitches are in between. Then keep track of how many rows you have done since then, and place st markers in the same place, and then finish your swatch and measure. Measure between the stitch markers, and use your notes that you took and do the math.