With filtering I was always taught the 30/30 rule (20/20rule in the UK because of the Mph vs Km/hr). That is if the traffic is doing 30km/hr or more then you don't filter and when you are filtering that the difference in speed between you and the other vehicles should never be more than 30Km/hr. It's not set in stone but it works well for me as a guide.
Of course on the motorway both here in Wellington as well as around Auckland it is difficult to stick to that religiously as the traffic speed tends to yo-yo quite a lot. Again it's a case of it being down to your own judgement at the time.
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