You could do things a few ways to avoid being booted:
Firstly, you could launch full-steam and back off after half track so that you don't go under 11's. This will give you a decent 60' time and also half-track time and MPH.
Secondly, you could break the beam on the green by creeping forward to trip the start timer...then launch the car and go as hard as you like. This will add to the ET but will give you the correct MPH.
You could then get an estimate of the car's potential by adding the best of both types of runs to see what the car is capable of.
Thirdly, go out for the final run and do a full pass and get booted
Then you could go back a couple of months later with different plates and do it all again :D