From the linky above:
Dean Margetts Achievements:
Debut – 2002 (Round 5 – West Coast Eagles vs. Brisbane Lions)
376 x AFL Games Umpired
12 x Finals matches umpired
So, our mate Deano has been at it since 2002. In his 19 years umpiring (I'll generously exclude 2021 for the purposes of this argument), there will have been 171 total finals games played. Each Finals game requires 3 umpires, meaning there were 513 total possible chances to be selected as a Finals umpire.
Dean was selected for 12 out of those 513 positions.
And of the 57 possible Grand Final umpiring positions in those 19 years, he was selected for 0.
Kinda tells you all you need to know about his level of performance when compared to his peers.