I like your calculations and hope all goes to plan. It's the slow riders at the back end of each race that need a hurry up-not riding to finish standing on the pegs in 2nd gear! (maybe a sweep rider to chase them up?)
20 mins before start of next race could be tightened up-remember that not all will be backing up for race after race-we leave that to Brad
At the Dirt Track car club I belonged to, we found that by putting a lot of prep in getting drivers for the next races all ready to go early tightened our schedule right up and we got through much quicker. (so a holding bay for the later races is a good idea)
As far as race length goes-they are national champs so make them work for it, both riders and bikes AND passengers.