Let's say we get sent out at 2 second intervals. When the last car gets sent, the first car will have left the pits 66 seconds ago. When the first car crosses start/finish, they'll be on their first hot lap, the slowest car still on their warm up lap.
If the sum the fastest car's warm up lap plus first hot lap is less than the slowest car's warm up lap plus 66 seconds, the lead car catches the slowest car on the first hot lap.
The best way postpone the front of the pack catching the back is to do one or both of the following:
1) Send the cars out closer together
2) The lead car should run a slow warm-up lap (taking care to warm the engine, tires, and brakes properly) so as to not spread out the gap even further.
...just my thoughts on trying to get everyone as many clean laps as possible