[Graham]

I had thought I would aim for a 5 lap / 1 minute round on my rally track. 

Having run a number of cars on it, I'm guessing that might offer a range of +/- 5 seconds for finishing times

I would agree with that, my original hillclimb track used previously on the 1st Road Race had an average 15-20sec lap time, and certainly 5 laps was quite a struggle getting a clean run.I have not tried the new one but it is a similar length of about 32ft long. My Carrera (Tarmac) stage is between 8-10secs.