First run at the track, time a guess?

If I go to the track with my Duster, new combo, and have no idea what it will run and don't have a roll cage, is there a calculation the tech's do to say I will need it based on my combo, or do I run and come in under the time required for a roll cage and they kick me out????? Only been the track once with my 318 and ran a 15.1... I now have a 422 and i will do much better than that, hoping for low 11's