You're not going to get accurate speed readings unless you actually roll the bike and measure the travel distance. And "factor" doesn't mean anything unless you know exactly what it is. It's not complicated math. It just wants to know your wheel diameter. It uses the magnet on your spoke to count each revolution. The measured travel distance formula is revolution x diameter x 3.14. the speed just takes the distance traveled and divides it by time.
Also the effective diameter decreases when you sit on the bike because the tire sinks under load and decreases the radius. So I sit on the bike and roll it 3 revolutions, take the distance traveled and divide by 3. My 2.1" wide tire is measured at an effective circumference of 1580mm at 3 bar pressure. If I don't sit on it, it's somewhere around 1590mm. Also if I decrease the pressure to 2.6 bar, I get something around 1550mm. This changes the speed and distance reading by up to 2%.