Here's the easiest and most accurate way to find your tire circumference:

Put a small piece of masking tape on your tire.

Position the tire so that the tape is at bottom-dead-center.

(If you can sit on the seat while doing this, it's even more accurate as your weight compresses the tire.)

Have an assistant put a corresponding piece of tape on the floor to line up with the tape on the tire.

Roll the bike forward in a straight line until the piece of tape on the tire is again at botton-dead-center.

Mark the floor again with another piece of tape.

Measure the distance between the two pieces of tape. Convert to mm (in * 25.4 = mm) That's the number your computer is looking for.