How do I calculate my own speed factor?

Your speed factor is, in broad terms, a ratio of how fast you are compared to the fastest person at the rally. So first, convert all your stage times to speeds in miles per hour. Throw out any that are abnormally low. Then average all of those. This is your representative average speed for the rally.

Now do exactly the same for the fastest person at the rally: convert all their stage times to speeds, drop the abnormal ones, and then average all the remaining. The result is the "fast reference average speed".

Then it's just your representative average speed divided by the fast reference average speed, and that's your speed factor.