Mark 1234
As Ando says, common sense IS the answer. That's why the original CTAF showing dimensions was not common sense.
The original TDN CTAF was 3 miles 3000' from memory. If they are dropping tandem paras from 4500' (thus not in CTA) then the old dimensions were unsafe as you would fly straight through them. If you are coming from MB (have a look at the chart) you go from ML Radar to a 3 Nm CTAF just in time to punch thru the paras again.
So, in your flight planning, you are wise to look for symbols like parachuting and also listen on the area and CTAF well back, setting a minimum dimension of 10Nm for a busy CTAF.
The airfields in proximity issue continues to be debated. The best advice on this is Annex E to NFRM 0401AS which states:
Overlap situations.
In some circumstances, an aircraft may be ‘in the vicinity’ of more than one non-controlled aerodrome having different frequencies at any time.
Where this overlap situation occurs, pilots of aircraft that have only one radio should select the CTAF of the aerodrome to which the aircraft is inbound, or if the aircraft is overflying, then the CTAF of the aerodrome to which the aircraft is nearer at any time should be selected.
Even then, commonsense is still needed - technically if you fly past Avalon you should leave the AV CTAF R frequency and transfer to Werribbee 126.7 when closer to Werribee. I don't recommend it!