The following is a Mackerras pendulum for the Victorian state election, 1996.
"Very safe" seats require a swing of over 20 per cent to change, "safe" seats require a swing of 10 to 20 per cent to change, "fairly safe" seats require a swing of between 6 and 10 per cent, while "marginal" seats require a swing of less than 6 per cent.
References
Post-election pendulum for the Victorian state election, 1996 Wikipedia(Text) CC BY-SA