Time improvement percentages

Formula question: If I have a list of times from races, how do I determine the percent improvement? Example (minutes, seconds, milliseconds) race 1 - 30:00.607, race 2 - 28:36.055, race 3 - 25:30.000 how do I display the percent improvement from the first to third race. There is about a 4 minute 30 second improvement in this example but sometimes there are no improvements or even a gain in time. If it were a gain, meaning it took longer, I think it should show as a negative (I’m not sure).