Depends on your riding and the weather and what tyres you have on. CB is to confirm if there is a tyre guy there, if so, he tyre guy can advise. Many others will also be able to give you advice. If you are a novice group rider and riding to the circuit, you may actually consider leaving them as they are or just reducing a couple, inters/faster riders may want to be around the 30:30 mark, but you will get others saying more in the front, more in the back etc. For novice and slower inters speed, to be honest it won't make much difference and it is good advice to run what you normally ride at as you will be used to how this feels, although if very hot on the day, your tyres will heat up more than normal. A sudden large drop will be very noticable and may feel very wierd, particularly if you then don't ride fast enough to get the tyres up to optimum operating temperature.
I run my Super Corsa Pros at 30:30 (cold), but my new Dunlops are recommended at 29:27 HOT I think (need to check that, but whatever, it's quite a lot lower with some of the new tyres coming out)
Fact is, you'll have loads of time to experiment and loads of people on hand to help, incuding Ducati and Hacketts, so can't really go wrong