‘You don’t have to be the best side all year, just the best side in September.’

This is a turn of phrase we hear annually, but each year the message is overlooked in the lead-up to the AFl grand final.

Like many others, I’ve felt that Hawthorn is a shoe-in for the flag for about six weeks now. But consider this: so far the Swans have had the better finals series. For the last five years or so, I’ve been successful in tipping the Grand Final winner. My tip has always been decided by a simple formula that suggests who has been the better team in finals.

I’ll explain how it works. The AFL finals series generally plays out the same way each year. Of the top-four sides, the two that win in week one progress straight to the preliminary finals. Barring upsets, the two top-four sides that lose in week one make it to the prelims, but not to the grand final.

There have been exceptions – such as the Brisbane Lions in 2003 – but not many, and none in the last few years. If the finals play out this way, as they have this year, we are faced with a familiar situation. Both grand finalists have defeated the same two sides to get there.

This year, for example, grand finalists Sydney and Hawthorn have both defeated Adelaide and Collingwood in their two finals. By comparing the results of both teams against two challengers, we can get a pretty good read on the form of the two grand finalists.

This is where the formula comes into play. Adding together the two winning margins for each team’s finals gives us one total margin across the finals.

Let’s take last year’s finals series as an example. Geelong defeated Hawthorn by 31 points in the qualifying final, then West Coast by 48 in the prelim. If we add these two margins together we get a 79-point total margin.

Collingwood, the other grand finalist, beat West Coast by 20 points in its first final, and snuck over the line against the Hawks by just three. The total winning margin across Collingwood’s finals series was 23 points – 56 short of Geelong’s. Collingwood was the best side all year, but Geelong won the flag.

Similarly, in 2010 Collingwood and St Kilda both defeated Geelong and the Western Bulldogs to qualify for the grand final. Collingwood’s total winning margin was 103, St Kilda’s was 28. The grand final was a draw, but Collingwood won the replay convincingly the following week.

2009 had St Kilda lose only two matches in the home and away season, and finish top of the ladder at the end of the home and away season. The Saints played Geelong in the grand final. Both teams defeated Collingwood and Western Bulldogs in the finals. St Kilda’s total winning margin across both finals was 35. Geelong’s was 87. Geelong won the flag.

The 2008 grand final is remembered as the one Hawthorn snatched from Geelong. Geelong had done everything right that year up until the grand final. Or had they? Their total winning margin across finals was 87 points. Hawthorn’s was 105 points. As we know, Hawthorn famously won the grand final.

In 2007 Geelong defeated Port Adelaide by a record margin in a grand final, but just to show that the formula held up in this year too, here are the numbers. Despite an incredibly close preliminary final, Geelong had a total winning margin across both finals of 111. Port Adelaide’s was 90.

Where does this leave us for this year?   If we add Hawthorn’s winning margins in its two finals together, we get a total of 43 points. This is made up of the Hawks’ 38-point victory over Collingwood in week one, and their five-point nail-biting win over Adelaide last weekend.   Adding the margins of Sydney’s two comfortable wins, having defeated Adelaide by 29 and Collingwood by 26, we get a total of 55 points – 12points better than Hawthorn’s total.

It’s not an enormous difference, but nevertheless, the numbers show Sydney has had a better finals series than Hawthorn, and will head into the grand final with a superior form-line. As we’ve seen in recent years, this is a huge advantage.

Perhaps the writing is on the wall? Sydney will no doubt come into the game fresher than Hawthorn. While the Swans got the job done fairly easily last week, the Hawks really had to grind out their win against an Adelaide side nobody really expected to be competitive.

If nothing else, last week’s match showed the Hawks are vulnerable. Sydney might just be the side to knock them off. They certainly won’t have any problem being the underdog – they’ve been an underdog all year.

If the Swans make the most of their chances today, they could deliver the biggest grand final upset since – somewhat ironically – 2008, when Hawthorn defeated Geelong.

Four years ago Hawthorn had more momentum going into the grand final than their competitor. This year they simply don’t.

Sam McInerney is currently completing a Graduate Diploma in Journalism at La Trobe University. You can follow him on Twitter: @SamuelMcInerney