I’ve noticed that with my practice lap times, my fastest lap is usually not the one where I’ve got the ideal set up on the car. Any ideas why this might be?
If it's within a tenth or so it is just that the drivers rarely do 2 identical laps. If you leave all the settings alone and do five practice laps you will find a spread of times. My advice is just use the practice laps to get the correct settings and work out fuel loads for each stint. Make notes this season because assuming you don't change your drivers and the weather is the same (regarding wet or dry) the settings will be the same next season.